Okay, you’ve already put one strap button on the tail end of the guitar. At the neck end, there are about five options:
This position is a popular spot: on the back near the heel cap, like on a Gibson semi-hollowbody.
Some folks place it right on the heel cap itself. Strap buttons in these spots 1 and 2 feel natural to electric players, but they allow the guitar to tip forward while playing. Also, when the guitar goes in the case, its weight will rest on the button, so I’d want deeply padded plush to support it in the case.
I like position #3 on the guitar’s side near the neck heel, because the guitar hangs comfortably, balances well, and doesn’t fall forward. And the strap’s out of the way of your fretting hand. If the neck block is wide enough, all you do is drill a hole and screw on the button. I install it centered on the side, about 5/8" out from the heel.
When the neck block's not wide enough, I glue a reinforcement block inside, where the side and neck block meet.
Carefully drill the hole through the side as shown with my see-through demo guitar. Then put glue on the block and use the button screw itself as a clamp while it dries. I use Franklin's Titebond for this type of job because it's strong and cleanup is easy.
Modern acoustics often have lag bolts fastening the neck to the body, so check inside before drilling to make sure you’re not going to hit metal hardware.
This is also good, right on the heel of the neck itself. What about tying the strap to the peghead? Don’t do it. You don’t want that kind of strain pulling at the neck that you’ve so nicely adjusted the action on.
This is the most popular choice among my customers. With the strap wrapping down under the heel, you'll like the way it pulls the guitar toward you. Surprisingly enough, it isn’t much in the way of your fretting hand either.
For a heel mount, use a strap button felt washer to avoid blistering the finish where the flat strap button meets the curve of the heel.
Use the right drill size
The bit should be comfortably larger than the core of the screw, leaving just the tips of the threads to cut into the wood. Use this photo as a guide: this is StewMac’s strap button bit and tap set. The bit and tap are shown overlapped in the middle, showing how your drill size should relate to the screw. This drill and tap set is what I use, so I don't have to hunt or guess at sizing. It's a good fit for all of these strap buttons: the Gripper, a traditional button, the Lockstrap System, Dunlop’s Straplok, and a pretty snakewood button.
Here's a trick that I learned from a customer — installing a strap button on the neck plate of a bolt-on guitar!