Here are a few Library links to get you started: Litter Training
The basic way to do it is to place the litterbox in the corner where bunny usually pees. Put some soiled litter into the box, and clean the rest of the cage.
Varying the type of litter between the box and the rest of the cage can sometimes be helpful Eg. blanket on floor, aspen shavings in litterbox.
All stray poops should be tossed into the litterbox as you find them.
Some buns like to have a hay snack while in the litterbox too
There have been a number of threads about littertraining recently... you could browse through this section and read the other suggestions in some other threads too
Remember, a young bunny is just a baby... it usually takes them awhile to figure out the litterbox