Step 1: The harness is attached to the backplate using velcro, detach the harness to reveal multiple harness size markings stemming from 1-10. Use these markings as a guide and test your way to the correct setting. Taller or bigger people usually find the best setting around 5-10 and shorter people usually find a good fit at 1-5. For women the harness settings could be slightly different. Always try aligning the buckle under the chest for optimal support and if you have a larger chest try using a larger harness setting.
Step 2: Once you've found the perfect harness setting you're almost done. Now all you need to do is tighten the chest straps until comfortable, close the waist belt buckle and tighten the straps to your pleasing, the straps are elastic and should give a smooth and comfortable fit.