Don't use the STL -- It's just there so Thingiverse is happy.
Tested with Marlin firmware, but no special codes are used.
Use Nylon filament; Taulman is best https://www.amazon.com/s/ref=as_li_ss_tl?url=search-alias=aps&field-keywords=taulman+nylon&linkCode=sl2&tag=poin0a-20&linkId=eb826af5361352a063300358c9bb8b94 -- I use 618.
Each line is documented

I was tired of the manual steps, so I coded up this routine:
Go to 230c, extrude 30 (to load filament -- Taulman Nylon 618)
Go to 100c, while extruding another 30 very slowly (to keep it solid as the temperature falls if your nylon isn't perfectly dry).
End...

Then you manually release the filament drive and pull the filament out the top; it will carry the gunk with it. Repeat until you get a clean pull.

To use:

1. Remove old filament (may need to preheat to do so)
2. Load Nylon (or at least get it ready; see 3b)
3. Execute g-code; it will
   3.a. Heat to 230c (where nylon extrudes freely on my printer)
   3.b. Extrude 30 cm to complete the loading and squirt a little out
   3.c. Cool to 100c while extruding another 30cm very slowly
   The printer should stop extruding at some point because it got too cold; this is to prevent puffs of steam from messing things up if your nylon isn't 100% dry.

Now, snap off the extruded part below the nozzle, then release the drive and pull the filament out of the top (this will require a good bit of force if you're not using a lubricated nozzle). The nylon will trap and hold the misc junk that was clogging the nozzle or making it difficult to feed.

Snip off the end and repeat as necessary to get a clean result (if I've had real issues it's taken 3 cleaning cycles to come clean). If you see the wick end, that means that the nozzle is completely clear and you can lubricate it if you wish (I recommend WS2 or HBn spray).