Many acoustic guitars are fitted with only one strap button on the tail end of the guitar. On this style of instrument, the opposite end of the strap is meant to be tied at the top of the headstock. A second strap button my be added in the area where the neck meets the body, but this requires drilling into the guitar, which must be done by a qualified technician to prevent damage. Most straps include a leather cord or string for acoustic instrument attachment, but you may use a suitable substitute if yours does not.
Attach the body-end of the strap to the guitar by placing the hole in the strap over the strap button.
Hold the guitar with one hand while standing and sling the strap over your shoulder, extending the unattached end to the end of the guitar neck with the tuning keys. Adjust the strap if necessary for a comfortable fit before attaching. Some straps must be removed completely before making adjustments and this will save the removal step. For straps with sliding adjustments you may skip this step or adjust it at this point for convenience.
Thread the cord or shoestring through the hole in the strap and pull it through until both ends are even.
Insert one side of the cord underneath the guitar strings right behind the white guitar nut and pull it through several inches clear of the guitar headstock on the opposite side of the insertion point. Inserting underneath the strings in this area will not interfere with tuning, and will prevent the strap from coming off accidentally after it is affixed.
Tie the two ends of the cord together with a string double-knot of your choosing and pull taut. Test the knot's integrity by pulling on the two sides of the cord. If the knot slips, use a different knot or strengthen the one you have tied.
Periodically check the knot for tightness, and the cord and strap-hole for fraying or wear. Leave a few inches of slack cord between the headstock and the strap-end. The bulk of the strap end should not interfere with the tuning keys, and the cord should not be wrapped tightly around the guitar neck. You may wish to consider a commercial leather strap attachment or quick-release tying mechanism instead of a cord. These are available at most music retailers.
Avoid using plastic, vinyl or other synthetic materials for the cord, as the chemicals may react with the guitar finish over time and cause discolouration. Resist the temptation to install a second strap button, or have the job done by a qualified repair person. Drilling into the guitar in the wrong spot can cause serious damage to the instrument.