There are two ways to get onto the rear tire that I have found. This is universal with mountain biking, trials and bmx's.
Method 1
1- Stick you butt way back over the rear axle. At the same time of sticking your butt back lift the front tire.
2 - To keep the momentun 'hump' your seat. Yo really have to be far back and you can't be afraid of falling backward.
method 2
1 - do a pedal kick to get the fron of th bike up. Essentially do a 1/4 power stroke to thrust the front end up.
2 - follow part 2 of above
One thing you need to be EXCEPTIONAL at is wheelies. To do a manual well you need to know you perfect balance point AND know how to feather the brake AND know how to hump to keep the balance point. Good luck.