start by inserting the tube into the side with the rounded fillets with the corner of the tube first to make it easier to insert.

this has a one way valve system that theoretically only allows the tube in and keeps it from reversing direction. but... it slips a lil in reversing. but... all you have to do is squeeze the middle of the toothpaste squeezer to secure the tube from moving back out. this hold to the tube pretty good.