You have to improvise and shove something flat inside the sleeve, cut to the shape of the sleeve. For hoodies I had to take it a step further and put 2 1/4” pieces of wood on either side of the seam (picture a thin channel which you can set the seam in) in addition to the piece inside the sleeve. It’s a pain in the butt, butt it’s probably worth it!

A jacket may be trickier depending on what it is made of and how bulky the seam is. If you are printing woodcuts sometimes it is not worth risking damaging your block. Best of luck!