Establish a true reference
Align the ruler with the grain, selvedge or a measured centre line before drawing. A straight line relative to a crooked edge can still produce a twisted finished pattern.
Hold the ruler firmly without pulling the cloth. If the fabric slides, secure the paper or use temporary support rather than increasing pressure.
Mark in manageable sections
Work from the centre or a stable corner and check each repeat as you move. Long uninterrupted lines can magnify a small slip across the whole panel.
Use the ruler to set patch margins and border breathing room as well as stitch lines. The empty space around a motif is part of the final design.
Check the result before stitching
Measure opposite edges, a few grid cells and the relationship to seams or hems. Correct the transfer while the cloth is still flat and unstitched.
Clean the ruler after marker work and keep it with the project plan. A marked edge or worn scale can create confusion on the next piece.