How to consistently hit Excellent throws?

Excellent throws sometimes happen by chance, or you need to throw the ball well. The way to throw well is with a curveball. If you're right-handed, throw it so it curves to the left. If you're left-handed, throw it so it curves to the right, and it'll pop up well. (I got Excellent throws 3 times when I got the legendary hat this way, and caught it with the last ball.)

Xlen is really about luck. It's tough unless the circle is pretty big.

You'll get better if you practice by giving Nanaberry in the wild.

If you lock the ball, it becomes a bit easier to hit Excellent throws, but since the catch circle size and distance vary for each Pokémon, you'll quickly get the hang of it with practice, as the commenter above mentioned.