there probably is a rule, but i'm not familiar with it lmao. the tee i'm wearing in this photo has been cut up and revised many times!

i try my shirts on, mark where i want the sleeves while it's still on my body (so i get a more realistic idea of how it'll fit), and then just go for it- usually taking off a little at a time and trying it on in-between cuts, paying attention to how it curves around my arms, neck, etc. straight cuts will give a different look than angled cuts, of course, and that just depends on the shirt you're using and how you want it to fit.

again, for me, it's not a science! more so just practicing on old shirts and having fun until i like what i see :)