The Strapkeeper is designed for the typical endpin jack buttons used by Fishman, L.R.Baggs, K&K, Switchcraft and others, but not the smaller diameter jacks found on factory installed systems used by Yamaha, Takamine, Ovation and Taylor Expression system, or the Vintage Jack. (See pics for examples of the different types it suits - strapjacks not included.)
Slide the leather Strapkeeper through the hole in your strap. Fold the Strapkeeper and slip the loops over the endpin strap jack. Push the non-marring nylon retainer into the groove around the end pin jack until it 'clicks'. Finally rotate the nylon retainer so that the opening is away from the guitar strap. Remove the nylon retainer by pulling on its keeper cord. If the retainer is especially snug on your strap jack, push on the open ends of the retainer while pulling the cord.