Cut the pipe square, preferably using a ratchet type scissor, pipe cutter or stanley knife, not a hacksaw!
Place the pipe in the mouth of the fittings, insert it in past the biting edge whilst pushing the pipe and the fittings firmly together, wiggle the pipe and fitting together.
The pipe will literally ‘walk’ into the Mul-T-Flo fitting. If possible, once the fitting is on the pipe and the pipe is seated at the stopper, turn the fitting through 360° to ensure that the biting edge of the fitting ‘bites’ into the inserted pipe and seals.