Print a pattern or template at exactly the size it should be.
1 Teach it your printer
Every printer shrinks a little, and "print at 100%" quietly lies. Print the test sheet, measure the line on it with a real ruler, and type in what you actually got. Do this once — it holds for that printer.
Correction: 1.000
2 What are you printing
A photograph of the part, a scan of the pattern, a screenshot of the plan. If it is bigger than one sheet it gets split across several with overlap and joining marks.
Drop an image here
or tap to choose · nothing is uploaded
3 How big should it come out
Give one real-world measurement and the other follows. If you know a feature in the picture is a certain size, measure that rather than the whole image.
How it works: a PDF carries real physical dimensions, so the file this builds is correct by construction — the trouble is at the other end, where your printer scales things by a percent or two without telling you. That is what the test sheet is for. Print it, measure the line, and every page after that is corrected by the same factor. When you print, choose Actual size or 100% and turn off anything called fit to page or shrink oversized pages, or the driver will undo the correction. Big jobs are tiled across sheets with an overlap and a corner triangle on each: line the triangles up, tape from the back, and the seam disappears. Everything is worked out in this tab and nothing is uploaded.