Wash the fabric – you want to do this for several reasons. The most important two are that newly bought fabric would shrink after the first wash, and the second is to remove any residue that may weaken the glue. Avoid fabric softener for this, as its chemical structure reacts to the adhesive and weakens the bond;
Choose a flat, smooth surface to work on. It will avoid crumpling and twisting of the tape;
Gently unroll the tape to keep it straight and avoid twisting it;
Measure the hem length to determine how much tape you need;
Apply the tape between the two hem surfaces, right along the folding line;
Begin ironing. Press gently for a few moments and release. For a short hem, slowly run the iron in a single slide along the hem. For longer hems, work in sections and check the tape placement;
Finish up the edge: you can leave it as it is, as hemming tape usually "seals" the raw edges and creates a nice finish. If you want to sew it, though, you can do it in any way you like.