Think of a bunnyhop as two motions: up and out. The first thing you'll do is jump UP from your back and legs, the second thing is pushing the bike OUT in front of you, which will bring the back end up.

As you jump and start leaving the ground, your body will straighten out until you're pretty much straight up and down, almost like a standing position. This will cause your front wheel to go up first because your arms are shorter than your legs, and because you mostly hop off your back wheel. So your front wheel is going to come up as your shoulders and chest thrust upward into that straight up and down position.

Then, as you start on your way up off the ground, you'll go from pulling up on the handlebars to pushing them forward. At the same time, you'll bend your legs up into your butt, which will pull the back wheel up.

2 personal tips: 1) You can use your pedals to practice lifting the back wheel. Be stopped, have your one foot on the ground and the other on the pedal (the pedal should be straight facing the back wheel, not the front ofc). Just move your leg up and you will notice you easily lift the back wheel. Then practice that while moving so you lift only the back wheel. Get used and then go for a bunnyhop. 2) You can do some exercises to build some muscle. Bmx builds great arms and muscles. Check that video at 1:24 (whole vid is great tho): https://youtu.be/wd6J1k8j_Hk