Lately questions about interfacing have been asked in the sewing classes. Students wonder whether or not to use sew in or iron on interfacing, what are the proper kinds to buy, and how to apply it properly to the fabric. In Sew 1, we had a discussion on the benefits of iron on interfacing vs the traditional sew in. The interfacing that you sew in is usually used for very technical construction projects like a blazer or a coat. Sewn interfacing is used in couture garments like gowns and women's suits, and has the same stabilizing effects as iron on, but requires more time to sew it in using special hand stitches. A great book that explains this in depth is Claire Schaeffer's Couture Sewing Techniques. It has amazing instructions and illustrations. And if you really want to learn hands on how to apply hand sewn interfacing, pick up a vintage blazer or a suit jacket from a thrift store and tear it apart. It will teach you about all of the different types and weights of interfacing used in these garments.
In general, for the hobby sewist, iron on interfacing works perfectly well. It should be applied to places like facings, plackets, collars, cuffs, waistbands, and lapels in order to keep the garments structure and to keep the precision of the design in place after multiple washings. For those like myself who prefer more aged and worn affects to appear in their garments, don't use interfacing! It will allow the fibers to follow their natural drape after washing.
The proper way to apply interfacing is to cut it out of the pattern from the same part that needs the enforcing. It can make transparent fabrics look opaque, which is especially helpful for white tailored shirts or lightweight garments where you don't want to see the seams in the more detailed parts like cuffs, fly, and collars. You can cut it out at the same size, or sometimes to avoid too much bulk you can cut it just within the seam allowance. This applies especially to facings where you need to finish the edge with a rolled hem. Just iron it on to the wrong side of the fabric, making sure to avoid creating any added puckers or pleats, and smooth it out as you go along. If you do create a pucker, just tear it off and start over. You will still be able to iron it back onto the fabric as you did before.
Hope that helps to answer some of these questions. Overall, interfacing when used properly, is an essential tool for creating a professional and polished garment.