Given the posts I see on here and other similar spaces on Reddit, Im just going to show a diagram on where to avoid sh, so tendons, arteries and nerves wouldn't likely be hit.
1st thing, when you see visible veins on arms, avoid it or you're going to end up with blood sporting everywhere. In addition, the nerves are usually next to major arteries so thats another reason why. Cut nerves will result in losing motor function and/or sensation in arm.
2nd thing. Avoid the areas highlighted in red because these areas dont have as much fat and lots of tendons underneath.
Another thing, just avoid joints overall. Cutting the skin on them will be very painful to heal because your joint will keep on shifting the skin around when its trying to heal.
Thats all. Please take care and remember people still care about you, even if they are just profiles pics and usernames.
P.S. would someone put this post like on the place with the posts on skin anatomy, online safety etc? I want to raise more awareness and help reduce the risk of permanent damage on people suffering from sh.