Very easy, use events toolbox and the "Layer motion" section:
http://wohlsoft.ru/editorhelp/manual_ed ... yer-motion
Note, there are blocks, BGOs, warps, water environments, and some NPCs were can be moved with layer (these are not free-falling, be aware old donut blocks, there is a bug)
You can call another event to change the speed or turn the move backward, etc. Use this wisely!
Note: on huge levels, if you use horizontal layer moves, you will cause built-in optimization to be disabled and on slow computers, everything will lag. Here is explained, why:
https://wohlsoft.ru/pgewiki/DrPepper_Problem