Edit: Note the new editor version for Saira 1.1!
Download editor here (to avoid spoilers, play through the game before you use the editor). Extract it so the editor program location is Saira\Editor\Editor.exe - the editor should be used with the Portable (zip) version of the game. Level editing is difficult and
completely unsupported. Please don't ask me for help, but feel free to help each other out in this board! Also be aware that custom levels only runs in the full version of the game. If you create a really good level that I think can give the game more attention, I can unlock is to that it plays from the game demo.
To make level editing easier, there are a few ingame debug keys in the game that's good to know. One can be considered a cheat.
F9 triggers a song instantly
F11 activates collision mask mode (hold Shift to display collision mask and sound emitters)
Hold F12 to move the player to the mouse, or to instantly travel to a star