Well, I have ironed what feels like a million little 3″ squares of fusible interfacing to a variety of different fabric scraps. I have learned a lot, but I sure feel stupid because I thought I knew everything I needed to learn at some point in my sewing career.
First, it did not seem to make a difference what temperature the iron was set on, and it did not seem to matter if it was a dry or steam iron. I tried using a damp cloth on a few samples, and learned that that is only a good idea if the fabric can handle it. Pre-shrinking the interfacing did not make a difference either, but my interfacing may have already been pre-shrunk.
There are two points I learned that are of the most value to me.
1) Do not slide the iron while applying fusible interfacing. Sliding the iron seemed to produce a lot of bubbles and wrinkles on the fabric side. Just set and lift the iron over the interfacing. Now, I am sure that I have applied interfacing just like this before, but why I forgot, I will never know.
2) Trim the interfacing so that it is 1/8 to 1/4″ smaller than the fabric you are ironing it to. I knew this, but never paid it any mind. I have always just cut the interfacing to the size of the pattern piece and applied, but I noticed that with my samples that I got a better adherence if the interfacing did not come completely to the edge of the fabric.
Hopefully, this newly remembered knowledge with help me to apply fusible interfacing with more success.