Hello. I'm trying to play with my friend in a modded server created by me, and he doesn't have a minecraft account. I want him to be able to see my skin, and possibly be able to see his skin if he changes it, but I didn't understood how to setup a custom skin server. Can you help me?

Ok. Thank you. I'll try that.

By default, with the mod installed, your friend should be able to see your skin even if your server is in offline mode as you have your skin configured on your minecraft account.
To setup a custom skin server that allows users upload their skins is not that difficult but still complicated.
I would suggest you just let your friend choose a skin then have the skin file on both your minecraft instances. If your friend find minecraft is interesting, he might buy it.