wow it is so cool to get feedback! thanks!
if you understood how easy this was you'd laugh, but making a tutorial is HARD so bear with me
1. cut front of non-hoodie sweatshirt in half and cut off collar
2. fold down and iron (unless you are lazy like me)
FLIP
3. turn inside out and get friend to pinch the back while you hold it closed to make the back darts (at least i think that's what they are called) and sew
4. (this part is kinda complicated and can be skipped if you sew the darts all the way up to the shoulder) cut across back and UP around arms from armpit to shoulder
5. pin from center lining the shoulderblade strip up with the newly-narrower waist part and sew
6. sew around armhole
7. cut excess dart fabric out of back and sew together in the middle (cuz its not really big enough otherwise) then cut to pleasing collar-shape (unlike my ugly drawing

)
8. center and attach to old collar/neck part
9. wrap yourself up and pin with funky pin or use buttons if you are that talented

ok, i hope that was coherent
happy crafting!