Finlay,
if your plans are on a computer, adjust them to the actual size of your model and print them onto clean white paper, several sheets as required. Print in black, and use a quick print mode to save on ink, and get a narrow line.
Then I would use a "Prit stick" for photograph sticking, and stick the paper onto the sheet of plastic. If needs be, cut the printed sheet and adjust the cut outs to best use your plastic. Then carefully cut through the line on the paper to the plastic. The photo "Prit stick" glue usually allows you to peel the paper off without it leaving bits behind.
Note: cutting plastic – just make a careful cut run, then bend the plastic back and forth gently to break along the cut mark, and give a nice square edge for gluing.
Kimosubby.