Export & install your font
In Fontself, tap the Sharing icon (on the top right) and tap Export Font File.
Tap the Procreate icon from the sharing dialog:
This automatically generates & installs the font file into Procreate's font folder.
Now, in Procreate, tap the Actions icon > Add Text and tap the Aa button (the text options button on the right of the keyboard) and scroll to find the name of your font. That's it ;)