Animation castle is a little like flipaclip I basically basically with a frame that the animation begins with and make a frame thats supposed to be the end of the animation, then I put two between those beginning and ending frames. I would then use the onion skin—if theres any—to mix those two frames to make it look like a smear of movement.

On castle idk but for other animation apps and stuff maybe try like rotoscoping since thats what almost all deltarune fans do for animation :P

Jarona