Fusible web is available in rolls, similar to tape, in various widths, by the yard in the interfacing department of most fabric stores and in pre-packaged pieces.
To use fusible web, the fabric should be washed first to prevent fabric finishes from stopping the fusible material from melting into the fibers.
Always read the directions that come with the fusible web you buy! Test your iron on an inconspicuous area to prevent ruining your garment.
Fusible web is available in various weights. Use the weight that is closest to the fabric you will be fusing.
Although it may be tempting to use a heavy weight fusible on everything to make sure it stays "forever", it may seep through the fabric leaving a melted mess and it is difficult to sew through. Heavy weight fusible will gum-up the sewing machine needle.
Many types of fusible web come with a special paper backing. Patterns can be traced directly on the back side of the paper for appliqué work. I always save the paper after I peel it from the fabric, using it to protect the ironing board and iron, similar to a press cloth.
Never iron directly on the fusible web. It will melt all over your iron! Rolls of fusible web make an excellent addition to your emergency supplies for quick hem repair.