This is gonna sound stupid, but

How do you do a motion tween in Toon Boom Harmony 12? I can do it so easily in Flash, but not in Toon Boom. Every video I look up didn’t help me.

go to insert and select Keyframe and advance the Red maker on the time timeline to whatever Frames you want, make sure you right clicked the frame on the timeline and selected Extend Exposure first, then use Transform tool on your drawing on the screen, animate position, rotation or scale, click Playhead on timeline to test, once you understand that you can do most anything, and to make onion skin animation select show onion skin tool and click G to advance and make next Drawing on next Frame until you complete your scene

It is referred to as “interpolation” in this software. If you want to understand the process read though the section on Cut-out Animation. It is difficult to tell from your post how much you already know. There are many things involved and there isn’t a short and simple answer unless you are asking how to turn on Onion Skin (which I do not think is the case).

Thank you! I can’t believe I couldn’t figure that out!