Plan out the animations you want, and how many frames of animation they require.
(For example, walk left: 30, walk right: 30, jump: 5)
Make animations in the animation editor in such a way that each animation starts with an idle / T pose.
String them together in the animation editor like so:
frames 0 - 29: walk left, frames 30 - 59: walk right, frames 60 - 64: jump
Don't touch body parts in animations where those parts shouldn't move. For example, if you try to straighten the arms in an animation where the player's left hand twirls, this will have adverse effects, as animations will occasionally overlap (such as walk + activate object making the arms swing faster)