To use the Mintest Skin Converter you need a recent browser (Chrome, Firefox and Edge are fine while IE 11 is broken) and some skins.
The full README and the sources are on Github:
https://github.com/godly/minetest-skin-converter
It's quite easy to use and allows selecting which Minecraft body parts are used for the Minetest skin. The Minecraft skins offer two legs and two sleeves which may be copied and overlaid into one Minetest leg.
It also offers a simple 3d preview / WYSIWYG.
Everything runs in your browser. Download the sources to run it locally without internet access.
Have fun!
Screeenshot:
