Square the reference
Use the selvedge, a pulled thread or the fabric grain to establish a straight edge. Do not trust a previous cut if the cloth may have been stretched or washed unevenly.
Press or flatten the fabric gently and mark a true reference line before measuring the finished panel or patch.
Add the right margin
Include the space needed for hems, folded edges, patch support and pattern breathing room. A sashiko field that reaches the cut edge can become difficult to secure.
For a repair, extend beyond the thinning rather than cutting only around the hole. The margin should reach stable fabric and leave room for a tidy stitch finish.
Check before the final cut
Place the paper pattern on the cloth, align it with the grain and look at the motif direction. Measure opposite sides and diagonals if the panel needs to be square.
Cut with long controlled scissor strokes and keep the cloth flat. Store the offcuts with the project because they are valuable for thread, marking and wash tests.