My gf is currently working on the same thing. She comes from a road bike background and never really used her front brake until MTBing. One of the things that's helped her has been improving her awkward body positioning in steep terrain. We started on flat terrain and just practiced positioning her chest over the saddle and butt as far back over the rear tire as possible. Once you get more comfortable with that start throwing in some front brake stops. Start light and work your way up until you can almost skid the front tire.
Back brakes will also be more effective with more weight over the rear tire, but like many have said, the majority of your stopping power comes from the front brakes, hence why some bikes have larger front rotor.
I've also really loved watching GMBN videos on YouTube for skill lessons. Not sure if there is one for this though.
Best of luck!