Grab a short length of string. Unlike electrics, most acoustic guitars only have one knob to hook the strap onto. Because of this, you'll usually need to use a piece of string or twine to tie one end of the strap to the headstock. This string can be made from virtually anything as long as it's thin enough to fit under the strings behind the nut.
If you don't have any string handy, try using an old shoelace - they're usually about the right length and thickness and are surprisingly durable.
Attach one end of the strap to the knob at the base of the guitar. The first part of putting a strap on an acoustic guitar is the same as for an electric. Slide the hole on the end of the strap that is farther from the buckle onto the knob at the base of the body of the acoustic.
As above, be sure to orient the strap so that the buckle faces outward and doesn't dig into your shoulder while playing.
Slip the string through the hole at the other end of the strap. Because the acoustic guitar has only one knob, the other end must be tied to the headstock. Begin by passing the string through the hole that isn't currently attached to the guitar's knob (the hole at the end of the strap closer to the buckle).
Slip the string under the strings and around the headstock. Take one end of your string and pass it under the strings just behind the nut (the wood or plastic piece at the far end of the fretboard that keeps the strings separated). Bring the end of the string under and around the headstock. The string should ideally rest just behind the nut.
Secure the string with a sturdy knot. Next, tie the ends of your string together. If your string is exceptionally long, you may want to "double it up" to reduce the distance between the strap and the headstock. Use a strong knot (or knots) - you don't want this string to come untied while you're playing.
Test your strap and adjust as necessary. Congratulations - your acoustic guitar is now ready to play! Test your strap out by playing in a variety of positions (as above). Use the buckle to make adjustments to the length of the strap as necessary. Listen to the sound of your notes - the string tied around the headstock shouldn't muffle or otherwise interfere with the normal vibrations of the strings in any way.
If the length of the string is too long or short to allow for comfortable playing, you may need to untie it and adjust it to a suitable length.
Install a second knob at your own risk. Rather than tie their strap to the headstock, some guitarists choose to install a second knob on their acoustic guitar. Usually, the knob is installed where the neck meets the body (to mimic the arrangement of knobs on an electric). Only attempt this if you have experience modifying guitars. Doing this incorrectly can permanently damage the guitar by splitting its wood.