A copy of Human: Fall Flat
A file explorer
A photo editor
That's it!

#1. Open your game
#2. Choose what clothes you want to paint on
#3. Paint on the piece of clothes you want to edit
#4. Save the person and go back to the main menu

Then it gets a little more complicated

#5. Open your file explorer and search MainCharacter with the search bar (if maincharacter doesn't have the image files, go to CoopCharacter instead)
Please note: these files are NOT in the Human: Fall Flat files (for a reason unknown to me).
#6. Press on the piece that you painted and edit it in an image editor, you can import a logo or just draw
#7. When you're done drawing, replace the old image with your edited one
#8. Switch back to the game and go into customization and you should see your edits on the human

Enjoy your custom painted human!