Sorry for posting this in here, I didn't see a bicyling for dummies or area for beginners.

I bought a bag along with a tire pump, patch kit and tools. The problem is, I can't figure out how this bag attaches to the bike. It's a wedge shaped bag, has a small velcro strap on the smaller side and a longer strap on the other side. I've had this for a couple weeks now and I thought I would figure it out eventually. On my way home I saw a bike on a rack attached to the back of a car and it has a pouch the same shape as mine. It looks like the pouch attaches to the rear underneath of the seat. The smaller velcro strap attach to the seat post, but I can't get the longer strap with clip to attach anywhere under the seat to make it look like it goes there.

The bike is a Trek 7200, I bought it a few weeks ago. I'd hate to go to the LBS and look stupid, so I thought I'd look stupid here.