Okay, heres what I think. Even if you usually don't do bunnyhops in trials it's still very good to know how to do them. Especially if you ever get into jumping, and bmx style street riding where there's less bouncing and stopping.
Heres how to do a bunnyhop: First, roll at whatever speed you want(When you're learning you should probably go slowly), then crouch down low, and bend your legs Then pull the FRONT wheel up as hard as you can while you jump with your feet(just as you would jump from a standing position on your feet.) You weigh more than your bike(hopefully) so if you push off with your feet hard enough to get yourself off the ground, then the bike will follow. Once you pull the handlebars almost up to your stomach the bike will be in a very steep position. At this point the back wheel is usually still on the ground. Most people get to this point, then they start having trouble. They ask "How do I get my back wheel off the ground now?" It's actually really simple. All you do is "unweight" the pedals, and push foreward slightly on the handlebars. To unweight the pedals, don't take your feet off the pedals, but take all the weight off of them. Just lift up your feet so they are barely touching the pedals at all. If there's no weight on the pedals, then it's easy for you to rotate the bike forward in the air by pushing forward on the handlebars. Most people have a hard time understanding how to unweight. The best way to practice is to ride with both wheels on the ground, and lift the back wheel up without using the brakes. It's NO DIFFERENT than lifting the back wheel up in a bunnyhop. If you can lift the back wheel up while riding on 2 wheels, then you can bunnyhop. The only difference is that in a bunny hop you do it while the front wheel is off the ground. No matter how strong you are. You wont be bunnyhopping onto picnic tables anytime soon if your just beginning, but if you practice a few minutes EVERY time you ride, you will notice much improvement in the near future. The more you practice, the higher you will go. Your body will learn by muscle memory how to bunnyhop so eventually you wont have to think about all this. You will do it without trying. The longer you unweight the pedals, the higher your back wheel will go. The higher you pull the front wheel up at the start, the higher it allows you to push your back wheel up. Normally this will level the bike out in the air, however, sometimes you don't want to land with the bike level. If you are landing on a downhill slope you want to drop your front wheel lower then the back. To do this, follow ALL the steps above(dont try to shortcut through it) then after you have reached the peak of your jump, unweight the pedals even more, untill the back wheel is higher than the front. If you want to land on your back wheel in a manual or stall, you need to put weight on the pedals, and pull back on the handlebars. This will tip the bike back. Remember to not do that untill you reach the peak of your jump.
I might have put to much info in there, so it might be hard to understand. Hopefully it will be helpful. Let me know if something doesn't make sense. Good luck.