How to make animations look smooth?

[FlyingGrass]

Nearly all animations I see have very smooth sitting down and lying down animations. I don't know how to describe it. There's no twitching to the pose after the character finishes transitioning to the pose.

How do I remove the twitching after the pose transitions?

[WolfQueen]

you have to match the end of the transition animation to the start of the idle animation.

[FlyingGrass]

you have to match the end of the transition animation to the start of the idle animation.

I can't get the end of the transition animation to match with the idle animation, is there any way to copy frames from one pose to another?

[WolfQueen]

I'm pretty sure you can.
Try making a copy of the idle animation and animate the transition animation from there, making the start of the idle animation the end of the new transition animation by copy-pasting keyframes.