Look at most modern day hiking, backpacks or outdoor gear (including tents, stuff sacks, ski fixtures, belts and holsters) and you find a variety of nylon straps and plastic "quick-clip" buckles. When the straps are joined and threaded through the buckles, they are used to fit the packs to the body, restrain gear to the pack and tighten weight loads for easier carrying. For the buckles to be effective on the packs, they need to be threaded onto the straps so the straps remain in them when placed under stress and weight.
Hold the strap in one hand and the buckle end in the other. Separate the buckle into a male and female end. The male end of the buckle has a knob that fits into the female end's empty hole-space.
Slide the strap (making sure you have the proper width buckle to match the strap) through the buckle strap slit, making sure to keep the strap going under the buckle through the slit.
Pull the strap back through the slit and then loop it through the opposite empty side on the slit (the slit consists of a bar running across the buckle for the strapping, leaving two open sides). The result is a strap that goes over-under-over, thus creating the friction to hold it in place when under tension or weight.
Clip the female to male end of the clips together and pull the loose end of the strap tight through the slit, then let tension hold the strap and buckle together.