Step 1: Acquire magnets
I get my rare earth magnets from http://www.kjmagnetics.com/
Step 2: Arrange magnets
Step 3: Load rifle
You'll want to make sure the inner magnet(s) fit into the outer. It should be a snug fit, not loose. It should also not be so tight that it can't shoot back out.
Step 4: Fire!
Firmly push the inner magnet(s) into the outer and it should pop out the other end with greater force.