Right, here is my attempt at explaining this! You will need your strap, two swivel clasps and the slider (a).
Slide one swivel clasp onto the strap, with the clip side on the right side of the strap (b). Add the slider after this, feeding the strap above the middle line but under the outer sections of the slider (c).
Fold the short end of the strap underneath itself so that the slider is now caught between the strap (d). The strap should be lining sides together. Stitch across the width of the strap close to the short edge to secure the slider in place (e).
Bring the remaining short edge underneath the other end so that the swivel clasp and slider are down one end and the lining side is showing on the remaining end (f). Fold this remaining end over on itself and guide through the slider as before by going under the first edge, looping the strap over the middle section and dipping back under the other outer edge (g). Pull through this end until the attached swivel clasp and slider are down the other end (h).
Slide the remaining swivel clasp onto the open short end of the strap with the clasp end showing on the outer side (i). Fold over the end so that the lining sides of the strap are facing each other. Stitch in place close to the short edge of the strap.