I created this file so that I could mount a pen to the extruder motor of my MK3S. It simply slides onto the motor. The pen is attached with rubber bands in the provided slots so that it can slide up and down to account for slight bed level differences.

I use PrusaSlicer to generate the gcode for prints. You simply need an stl that is .2mm high. I will paste my config bundle below. I'm pretty sure all you need to do is create a new .ini file and paste it in.

To hold the paper in place, you can use either magnets or tape.

I have also created a new steel sheet named plotter. I used the smooth steel sheet and set the live Z to 0. This keeps the nozzle clear and lets me set the pen to be just below the nozzle. I have my settings do a Z hop of 2.5mm. You basically want to set the pen to not touch at 2.5mm but touch at about 1mm. It is pretty much guess and check on a scrap piece of paper.

Here is the config.ini file

generated by PrusaSlicer 2.3.0-alpha2+ on 2020-12-10 at 21:34:20 UTC

[print:MK3S Plotter]
avoid_crossing_perimeters = 0
bottom_fill_pattern = rectilinear
bottom_solid_layers = 5
bottom_solid_min_thickness = 0.5
bridge_acceleration = 1000
bridge_angle = 0
bridge_flow_ratio = 0.8
bridge_speed = 30
brim_width = 0
clip_multipart_objects = 1
compatible_printers = "MK3S Plotter"
compatible_printers_condition = printer_notes=~/.PRINTER_VENDOR_PRUSA3D./ and printer_notes=~/.PRINTER_MODEL_MK3./ and nozzle_diameter[0]==0.4
complete_objects = 0
default_acceleration = 1000
dont_support_bridges = 1
draft_shield = 0
elefant_foot_compensation = 0
ensure_vertical_shell_thickness = 1
external_perimeter_extrusion_width = 0
external_perimeter_speed = 25
external_perimeters_first = 0
extra_perimeters = 1
extruder_clearance_height = 20
extruder_clearance_radius = 45
extrusion_width = 0
fill_angle = 45
fill_density = 100%
fill_pattern = rectilinear
first_layer_acceleration = 1000
first_layer_extrusion_width = 0
first_layer_height = 0.2
first_layer_speed = 100%
gap_fill_speed = 40
gcode_comments = 0
gcode_label_objects = 0
infill_acceleration = 1000
infill_every_layers = 1
infill_extruder = 1
infill_extrusion_width = 0
infill_first = 0
infill_only_where_needed = 0
infill_overlap = 25%
infill_speed = 80
inherits = 0.15mm QUALITY @MK3
interface_shells = 0
ironing = 0
ironing_flowrate = 15%
ironing_spacing = 0.1
ironing_speed = 15
ironing_type = top
layer_height = 0.2
max_print_speed = 200
max_volumetric_speed = 0
min_skirt_length = 4
notes =
only_retract_when_crossing_perimeters = 0
ooze_prevention = 0
output_filename_format = {input_filenamebase}{layerheight}mm{filamenttype[0]}{printermodel}{print_time}.gcode
overhangs = 1
perimeter_acceleration = 800
perimeter_extruder = 1
perimeter_extrusion_width = 0
perimeter_speed = 45
perimeters = 1
post_process =
print_settings_id =
raft_layers = 0
resolution = 0
seam_position = nearest
single_extruder_multi_material_priming = 0
skirt_distance = 2
skirt_height = 3
skirts = 0
slice_closing_radius = 0.049
small_perimeter_speed = 25
solid_infill_below_area = 0
solid_infill_every_layers = 0
solid_infill_extruder = 1
solid_infill_extrusion_width = 0
solid_infill_speed = 80
spiral_vase = 0
standby_temperature_delta = -5
support_material = 0
support_material_angle = 0
support_material_auto = 0
support_material_buildplate_only = 0
support_material_contact_distance = 0.25
support_material_enforce_layers = 0
support_material_extruder = 0
support_material_extrusion_width = 0.35
support_material_interface_contact_loops = 0
support_material_interface_extruder = 0
support_material_interface_layers = 4
support_material_interface_spacing = 0.2
support_material_interface_speed = 100%
support_material_pattern = rectilinear
support_material_spacing = 3
support_material_speed = 50
support_material_synchronize_layers = 0
support_material_threshold = 55
support_material_with_sheath = 0
support_material_xy_spacing = 75%
thin_walls = 0
threads = 8
top_fill_pattern = monotonic
top_infill_extrusion_width = 0
top_solid_infill_speed = 40
top_solid_layers = 7
top_solid_min_thickness = 0.6
travel_speed = 180
wipe_tower = 1
wipe_tower_bridging = 10
wipe_tower_no_sparse_layers = 0
wipe_tower_rotation_angle = 0
wipe_tower_width = 60
wipe_tower_x = 170
wipe_tower_y = 125
xy_size_compensation = 0

[filament:PEN]
bed_temperature = 0
bridge_fan_speed = 50
compatible_printers = "MK3S Plotter"
compatible_printers_condition = printer_model!="MK2SMM" and printer_model!="MINI" and ! (printer_notes=~/.PRINTER_VENDOR_PRUSA3D./ and printer_notes=~/.PRINTER_MODEL_MK(2.5|3)./ and single_extruder_multi_material)
compatible_prints = "MK3S Plotter"
compatible_prints_condition =
cooling = 0
disable_fan_first_layers = 3
end_filament_gcode = "; Filament-specific end gcode"
extrusion_multiplier = 1
fan_always_on = 0
fan_below_layer_time = 20
filament_colour = #0F80FF
filament_cooling_final_speed = 3.4
filament_cooling_initial_speed = 2.2
filament_cooling_moves = 4
filament_cost = 43
filament_density = 1.27
filament_deretract_speed = nil
filament_diameter = 1.75
filament_load_time = 0
filament_loading_speed = 28
filament_loading_speed_start = 3
filament_max_volumetric_speed = 8
filament_minimal_purge_on_wipe_tower = 15
filament_notes = ""
filament_ramming_parameters = "120 100 6.6 6.8 7.2 7.6 7.9 8.2 8.7 9.4 9.9 10.0| 0.05 6.6 0.45 6.8 0.95 7.8 1.45 8.3 1.95 9.7 2.45 10 2.95 7.6 3.45 7.6 3.95 7.6 4.45 7.6 4.95 7.6"
filament_retract_before_travel = nil
filament_retract_before_wipe = nil
filament_retract_layer_change = nil
filament_retract_length = 1.4
filament_retract_lift = 1.5
filament_retract_lift_above = nil
filament_retract_lift_below = nil
filament_retract_restart_extra = nil
filament_retract_speed = nil
filament_settings_id = ""
filament_soluble = 0
filament_toolchange_delay = 0
filament_type = PEN
filament_unload_time = 0
filament_unloading_speed = 90
filament_unloading_speed_start = 100
filament_vendor = Generic
filament_wipe = nil
first_layer_bed_temperature = 0
first_layer_temperature = 0
inherits = Generic PETG
max_fan_speed = 50
min_fan_speed = 30
min_print_speed = 15
slowdown_below_layer_time = 20
start_filament_gcode = "M900 K{if printer_notes=~/.PRINTER_HAS_BOWDEN./}200{elsif nozzle_diameter[0]==0.6}24{else}45{endif} ; Filament gcode"
temperature = 0

[printer:MK3S Plotter]
bed_custom_model =
bed_custom_texture =
bed_shape = 0x0,250x0,250x210,0x210
before_layer_gcode = ;BEFORE_LAYER_CHANGE\nG92 E0.0\n;[layer_z]\n\n
between_objects_gcode =
color_change_gcode = M600
cooling_tube_length = 5
cooling_tube_retraction = 91.5
default_filament_profile = "Prusament PLA"
default_print_profile = 0.15mm QUALITY @MK3
deretract_speed = 0
end_gcode = G4 ; wait\n{if layer_z < max_print_height}G1 Z{z_offset+min(layer_z+30, max_print_height)}{endif} ; Move print head up\nG1 X0 Y200 F3000 ; home X axis\nM84 ; disable motors
extra_loading_move = -2
extruder_colour = ""
extruder_offset = -42x-48
gcode_flavor = marlin
high_current_on_filament_swap = 0
host_type = octoprint
inherits = Original Prusa i3 MK3S
layer_gcode = ;AFTER_LAYER_CHANGE\n;[layer_z]
machine_limits_usage = emit_to_gcode
machine_max_acceleration_e = 5000,5000
machine_max_acceleration_extruding = 1250,1250
machine_max_acceleration_retracting = 1250,1250
machine_max_acceleration_x = 1000,960
machine_max_acceleration_y = 1000,960
machine_max_acceleration_z = 1000,1000
machine_max_feedrate_e = 120,120
machine_max_feedrate_x = 200,100
machine_max_feedrate_y = 200,100
machine_max_feedrate_z = 12,12
machine_max_jerk_e = 4.5,4.5
machine_max_jerk_x = 8,8
machine_max_jerk_y = 8,8
machine_max_jerk_z = 0.4,0.4
machine_min_extruding_rate = 0,0
machine_min_travel_rate = 0,0
max_layer_height = 0.25
max_print_height = 210
min_layer_height = 0.07
nozzle_diameter = 0.2
parking_pos_retraction = 92
pause_print_gcode = M601
print_host =
printer_model = MK3S
printer_notes = Don't remove the following keywords! These keywords are used in the "compatible printer" condition of the print and filament profiles to link the particular print and filament profiles to this printer profile.\nPRINTER_VENDOR_PRUSA3D\nPRINTER_MODEL_MK3\n
printer_settings_id =
printer_technology = FFF
printer_variant = 0.4
printer_vendor =
printhost_apikey =
printhost_cafile =
remaining_times = 1
retract_before_travel = 0.5
retract_before_wipe = 0%
retract_layer_change = 1
retract_length = 0.8
retract_length_toolchange = 4
retract_lift = 2.5
retract_lift_above = 0
retract_lift_below = 209
retract_restart_extra = 0
retract_restart_extra_toolchange = 0
retract_speed = 35
serial_port =
serial_speed = 250000
silent_mode = 1
single_extruder_multi_material = 0
start_gcode = M862.3 P "[printer_model]" ; printer model check\nM115 U3.8.1 ; tell printer latest fw version\nG90 ; use absolute coordinates\nG28 W ; home all without mesh bed level\nG0 Z5\nM117 Drawing..
template_custom_gcode =
thumbnails =
toolchange_gcode =
use_firmware_retraction = 0
use_relative_e_distances = 1
use_volumetric_e = 0
variable_layer_height = 1
wipe = 1
z_offset = 0

[presets]
print = MK3S Plotter
sla_print =
sla_material =
printer = MK3S Plotter
filament = PEN
physical_printer =

Print instructions

Just slice and print, or use the provided gcode.