To start, use a rat-tail comb to create a part on the side of your head, like pictured here, and secure the top layer out of the way with a clip. Then grab the weft you want to use and attach it to a horizontal layer of hair from below the part. While other extensions may require you tease your hair at the root to ensure the weft stays in, my Beyond ones stay in without teasing. Use the middle clip first to snap it onto the hair, then snap the other sides close.
Image Source: POPSUGAR Photography / Sarah Siegel