When I did my medusa cosplay I used these: White Stickers
I just colored them black and stuck them on. They stayed pretty good except for the elbows but I would just replace them every few hours. Plus you get like 500 of them for $2.00. You can get them at walmart.
As far as the snake heads I did just sharpie those on, but at least this method keeps you from spending forever making small dots on your arms.
How they looked on me.