FOSS gamedev and creative worlds

OK So I already know that there are some mods out there that add skins and where you can chane skins but Im kind of confused. I need help on:

Supporting Inventory Mods (I used the crafting mod that adds the Minecraft texture for the inventory and a crafting table)

How to upload skins (once I get a skin mod, how do I upload a skin into the mod/world?)

I already know what website to use to make and get skins but I really need help! I would LOVE to be able to have my own skins and being able to change them in game. Please help! Thnx!

https://forum.minetest.net/viewtopic.php?id=7527
https://forum.minetest.net/viewtopic.php?id=4147
https://forum.minetest.net/viewtopic.php?id=8816

http://minetest.fensta.bplaced.net/

You should be able to see how to get the skins in the topic of those mods.
Those mods should store the skins in their folder, so just open it and look if you can add there you own skins.