I am afraid the answer is "With a lot of trouble".

Usually the spline is a little longer than the tranny shaft.
Clean the gunk out of here and then put some Liquid Wrench in.
Check it every once in a while and add some more if it is soaking down.
Leave it for an hour and then give it another try.
If it does not work repeat the above.
You may have to let it sit overnight.

I assume that you are grabbing it with vice grips or pliers trying to get it off.
Try moving it left/right then up.

Some people use WD40 but I find Liquid Wrench better.
If you work on cars you may have a gear puller that would do the job.
I find you will struggle with it and suddenly it will pop off.
If you have to cut it off a MotoTool (Dremel) with the thicker cutting wheels works though it will probably break half a dozen wheels unless you have a very steady hand.

Perhaps someone else will jump in with a better solution.