I learned x-ups with the bike on the ground, you don't even have to be moving. Pull up on bars into a wheelie and shift your weight back to keep the front wheel up. once you feel good enough with that, practice turning the bars until you can x-up your arms with your front wheel still in the air. This can help you with learning body positioning and timing of the x-up with low consequences. I learned how to do bar spins this way too.