press space to add a block, utilizing the width and height sliders press c to place an enemy(or entity), utilizing the enemy id slider look inside the project for more control. There you could do something like copy the blocks data and enemies data and put them in a text file, and then when coming back here you can use the load blocks and load enemies functions to use the data in the text file to load a level.
This has a short story behind it. I was working on a run and gun scratch project and considered making it using sprites and clones or only the pen for rendering. I chose the first option as it could let me make levels more easily just using costumes for each one. The problem however is I don't know how to do much complex stuff with scrolling for projects consisting on sprites and clones as objects(such as when acceleration's involved). So I thought to switch to pen only, but I could've faced tediousness with level design as all I had known on how to make levels for a pen only game is manually adding each entity in code. So I planned out the idea of this, and saw I could make it in a way that'd make it quite simple but effective for me. And now here we have it. At some point I think I'd like to make an actual level editor for my scratch games, but I'd like to use this as of now. I think I'd remix this for a few games of mine to use to make the levels for them, such as the run and gun game I mentioned.
