I've just made an in-game level editor for my current project, and while there is nothing too complicated about it, it's a reasonably large task.
I would suggest you have a go at making it yourself and then for the parts you get stuck with, ask for specific help on that.
Otherwise this request is too general to give a lot of help.
I can tell you I used a ds_map / ds_list solution to save my level to disk using Json.
There are plenty of tutorials on this forum on how to do that.