1. The guitars are usually installed with an strap nail at the bottom. Place the strap into strap nail the like this.
2. Place the strap into the strap nail on the neck or fasten it to the headstock.
For ukuleles, there are usuall two kinds of ukulele straps.
Like following picture, this kind of strap is similar to the guitar we mentioned above.
There is another kind of ukulele strap. Simply hook onto the sound hole.