For guitars with two strap buttons, it shouldn't be a problem to attach the strap easily. Problem here is the guitar with only one button on the butt, none on the front side. And this is usually seen on acoustic guitars.
So u'll need a string (shoe string is the most common but any strings that are thin enough to fit through will work well) to tie it on the headstock, and it's very important to tie it properly so that your guitar will not smash on the floor while you're playing with it.
Let see, I have a Ernie Ball strap here, and a shoe string.
There's two ways to tie the string on the keyhole of the strap. I'll show both of them.
#1
Run both ends of the string though the keyhole of ur guitar strap, tug on it.
This is simple but u may ruin the leather of the strap after a long period, so the 2nd way may be better.
#2
Run one end of the string through the keyhole, and tie a knot close to the keyhole.Hmm..Much better..
Next slip the shoe string under the guitar strings above the headstock, NOT on the neck please...ur guitar will buzz or even be muted.
While the other end of the string behind the headstock, tie a double knot on the end.
Final step.. stuck the keyhole of the other end on the butt. DONE!!!
Don't recommend to drill hole on guitar jz to fit another button, imagine how will u act if mistakes happen..LOL..
by da way, permanently modify ur guitar may jz cause the value to drop, so i don't see the point to take the risk.