1. When the clasp is opened, you can see a little arrow in the middle part. You can lift up that part using the adjustment tool (or a small, flat object). 
2. Slide the clasp to the desire size. (Please be aware of the little indentations on the back of the strap.)
3. Push clasp down to firmly secure. (Or use the adjustment tool to clasp down in the reverse direction). 
4. Pull the hook over the pin and snap the flat clasp closed. Hint: You should hear a click when it's fully secured.