how do you make "hold" levels

If you're talking about the levels where you just hold down on the screen and everything is timed perfectly to help you complete the level, then that's basically what it takes to create them - you have to carefully plan and test gameplay to ensure that, as long as a player starts holding when you want them to, they'll complete the level without any problems (this is because even just one error can make a level unbeatable so everything has to be perfect). We used to have to worry about different jump physics for different versions of GD but that shouldn't be a problem now as long as you're building in 2.1.