This repository contains the source-code for most of the levels included in JOY OF PROGRAMMING. If you are creating your own levels this might be a good starting point. Beware that these sources include spoilers about each level and might make it easier for you to solve them.
Generally I'd recommend uploading your own creations to the Steam Workshop for JOY OF PROGRAMMING. If want to contribute your level to the main game, you can do so under the MIT License by raising a pull request here. But please understand that I cannot guarantee to include your level in the game.