I may be a little late on this, but I'm always putting liners in helmets. I buy the el-cheapo ones from Home Depot (the ones without the wheel because I will only ever adjust it once. I put it on my head first and size it. I then tape all the cloth straps together in the center just to keep them nice and neat. My next liner, I will cut these off because they serve no purpose for me. My neck is so long, that the helmet needs to sit as low as possible, so the top of my head hits the top of the helmet anyway. The straps are just sitting in there.
After sizing (and possible taping), I fold the tabs up and tape them into the helmet first with masking tape. I put the helmet on and see how it sits. If it's wonky, I loosen the tape and reposition. Once it is where I want it, I mark off where the tabs are and remove the tape. I will then hotglue the tabs in place. I do not sand the helmet where the tabs go either. You will get a great bond if you do sand, but if the liner ever breaks (I've had this happen) you'll have a tougher time pulling it out.
Once glued, I go over each tab with a strip of industrial strength velcro (the fuzzy side) to further strengthen the connection between the tab and the helmet.