>>289437945
>bent backwards
As in the joints bend in the wrong direction? If so the elbows (and knees) should have a slight bend in them so the vrc IK knows which way they're supposed to bend.
>making the arms shorter
FBT works best if your avatar's proportions match your own real life ones. Search for Kung's fbt tutorial on youtube for a really good tutorial on all the fixes you (might) need to do. Something in there he says to do is to increase (or decrease) your irl player height setting before binding in which will make everything in the world smaller (or larger). It might help making things fit a bit better. Avatar arm length also does some weird thing with scale too so really long arms can trick the game into thinking it's a really big avatar and vice versa.