There are no tutorials of how to use this in an online server, nor how to use hd skins with this, can someone explain it for me

Its completely client side, but other people use this mod, can add your username to local, and set the skin. I think the HD Skin its just like optifine, you just put the HD Skin in .minecraft/CustomSkinLoader/LocalSkin/skins/<your username>

@barraIhsan There is lack of tutorials about how to load skins online in English.

There are some skin websites in default CustomSkinLoader load list:

• Little Skin
• Blessing Skin
• ElyBy
• SkinMe
• GlichlessGames

you can sign in one of them and select or upload your skin.