I normally place it around where the CG is. If it's split then I place each section equal distance away from the CG. Sometimes, like in the case of the Cherokee D, the launch lug is at opposite ends of the body which seems to work fine.
I'm a fan of getting it as far back on the rocket as you can, particularly on rockets that are only marginally stable. This way you get guidance as long as possible to get the speed of the rocket up before it's flying on its own.
limd--welcome aboard!! I too got started again with the kids, but, quite frankly, I've got the bug more than they do! Who needs excuses?
Balance the rocket on your finger...where it balances is CG.
Now, install the largest motor you plan on using into the rocket, along with the parachute, etc...as if it is ready to launch. This will give you the maximum Cg reading when balanced once again. this is important when designing stable rockets.
Unless the rocket is very small, I often use 2 LLs, placed high and low. Usually I just cut one in half. I always mount the LL on a sliver of balsa or other wood, like a thin coffee stir stick, just to get it up off the paint.