where on my bike do i mount the holder for my bike lock? (trek)?

[Jess]

so i recently got a new trek mountain bike, and a lock, obviously so it won't get stolen.
anyways, i can't figure out where i am supposed to mount the little holder at! it's probably somewhere obvious, but i really can't figure it out.
the circumference of the piece i'm mounting is fairly small, so it can't be any of the bigger bars of the bike. (sorry, this is hard to explain)
i don't want to look like and idiot with my lock wrapped and mounted on my bike in the wrong spot!
help me please!
thank you!

 

[reignofcheese]

Usually these holders are meant to fit the same bolts that the water bottle cage uses.

 

[Mike C]

It mounts to your seat post.