Prepare image of your choice. Square fragments yield the best results. You can cut them out e.g. in GIMP.
Load image from your disk into form.
Select the fill color of skin's sides.
Hit "Create skin" button. Download will start automatically.
In your Minecraft launcher navigate to Skins > New skin > Browse. Select your skin and submit.
Before running instance of the app, make sure to run configure script.
This will create Python virtual environment in pyenv directory, download necessary Python packages and create directories required for the app to work.
./configure
npm install
npm start