There is no way to attach a waist strap to a med sized Alice Pack, the strap attaches to the Frame Only. Large Alice packs can't be used without a frame, there is no way to attach straps directly to the Pack.
The loops on your shoulder straps attach to loops on the top front edge
of the Alice, undo the loops on the shoulder strap and thread them thru the top loops of the the pack and re-attach/re-thread the shoulder strap. Do the same for the bottom of the strap attachment points to bottom sides of the pack.
If you posted a image of your pack it would be much easier to describe.