Strap locks are cheap insurance to protect your guitar from falling off your body and crashing onto the floor. For about £13, you can purchase these handy little devices and, after their installation, play with near reckless abandon, never having to worry about the strap slipping off the strap buttons ever again.
Remove the strap. Take the strap off your guitar as you normally would. If you leave it on while changing strap buttons, it will just get in the way.
Remove the strap buttons. Using your screwdriver, remove the screw that affixes the strap buttons to the guitar.
Save the felt. There may be small circular pieces of felt under the strap buttons. Save them. You'll need them when you install the new strap locks.
Save the strap buttons. Take your new strap locks out of their packaging and put the stock (old) strap buttons inside. You never know when you'll want them again.
Know the parts. Strap locks have two main parts--the strap button and the locking device. Locking devices come in two basic styles: the horseshoe and the press-on button. A quick look at your strap locks should make it obvious which kind you have.
Put on the strap buttons. Using the screw that came with the strap locks (this is very important), apply the new strap buttons. Place the circular felt around the screw hole, place the strap button on top of it, and screw it in. Repeat for the other strap button.
Affix the locking devices to the strap. For the horseshoe strap lock, remove the nut and washer. Put the head of the lock through the strap hole so that the release device is pointing out. Replace the washer and nut, then clip the strap lock onto the strap button. The button strap lock is a little more difficult. If it is already assembled, take it apart by removing the retainer clip (the piece closest to the bottom). The rest should come apart. Next, push the locking pin into the washer with the large indentation. This will go on the outside of your strap and will fit inside the parts that will go on the bottom. Push the locking pin all the way through the standard washer and the retainer clip. These will go beneath the strap. The piece is now assembled.
Tighten and adjust. If you have the horseshoe strap locks, make sure the horseshoe's open end is pointed toward you. Failure to do this will cause the strap button to slip out of the lock's grip.
In a pinch, guitar players have been known to use everything from bottle caps to duct tape as strap locks.
You may find that the new screws are too large for the holes in your guitar. This will require a new or larger hole. If you drill, do it carefully. If you're not good with a drill, enlist the help of someone who is.