A basic template for drawing simple Celtic knot borders with four strands. You can use the holes in the template to draw and align to a dot grid, then draw the border within it. You shift the template the correct amount using the dot grid to align for the next element. Each section has a part which shows how the two, three, or four sections of the template will align to create the final design.

There is a simple knotwork border section, four possible terminals (short and long) and two possible corners. If people like this I hope to add more bits and improve it.

You may need to use XY compensation to make the grid holes large enough to use, depending on your printer.

To do lines of straight border, you draw one set of outer curves, then move the template one single grid square along the line, and draw the second set of outer curves with the same section of template. Then you move it over along the grid to place the cross pieces. 

For terminals and corners, it's best to align as well as you can with the grid and with the border already drawn, then fill in the four parts of the corner (or three of the terminal) in order. The first part of each section of the template has relief to show how the next parts fit into it.

Note: the little knotwork partial border shown in the photos was done very quickly, without laying out enough grid, and with a print that needed cleanup. Better results can be achieved with the model in its current state.