1) Don't forget to set your record buttons 1st!
2a) Practise just killing the opponent and getting timings down for when to act to mix the next character up.
2b) You cab lower their life % for this or set them to block/pushblock in different ways
2c) You can record them to mash footdive/helmbreaker/Gimlet etc on the way in to practise not getting beat out and a poorly timed mixup
3a) Once you have a rough idea on what your want your mixup to be, pick the P2 side team to be the team you are using, and then use the Record and Playback function to experience what the opponent deals with when they face your mixups.
3b) Then try your best to escape using movement options, to beat it using normals/specials/supers or defend it using block/pushblock/XF etc and see where the setup is weak
3c) Finally refine your setup in order to understand the small differences between making it hit left or right, to account for their pushblock, or to cover their escape attempts
4) Final option is to plug in a P2 controller and physically force it to walk one direction or the other, this way they will always block the same way and you will be able to test your left/right mixups freely.
I hope this helps, if you have any difficulty with these methods and can't figure out a way to test it drop me a comment below and I'll try to help as soon as I can. Thanks, and good luck mixin.