Start beyond the hole
Measure the thinning around the visible damage, not just the opening. The patch should reach sound fabric with enough margin for stitching and washing.
Choose a cloth with a compatible hand. A very stiff patch on a soft shirt can create a hard boundary, while a flimsy patch may simply move the tear outward.
Check weave and edges
A close, stable weave gives the thread something to hold. If the chosen cloth frays heavily, allow extra margin, turn the edge or use a suitable preparation method.
Place the patch over a seam or pocket only when the added thickness will not interfere with movement. Test the layers before marking the sashiko field.
Match the repair to the garment
Denim, workwear, knit-adjacent clothing and fine cotton all need different patch weights. The patch should reinforce the garment without making the repaired area uncomfortable.
Wash a small material pairing before a visible repair. This reveals shrinkage and colour change before the patch becomes permanently attached.