It was actually quite easy, and seemed to work with several different weights and types of fabric. Just take a small piece of fabric (bigger than your design) and a piece of fusible interfacing, iron them together as per product instructions, and then draw on either side and cut out your shapes.
A note of warning: make sure your piece of interfacing is at least a little *smaller* than your fabric when you're ironing them together, or you'll fuse the overlapping sections of interfacing to your ironing board.... oops!
This thistle design on a plaid flannel is the only one I've sewn on to anything so far, but I really like how it turned out. I used a sort of connected running stitch to secure down the edges, and I really like how it gives a solid line, and doesn't take away too much from the plaid. Often I like to use a blanket stitch on the edge of knit appliques, but that would have been too busy here. The very edges of the flannel did start to fray a bit, but only after I'd sewn them down - the interfacing held everything together. Also, I feel like between the interfacing and the stitching, it's very secure. The bonus is that it took much less time than a satin stitch around all the edges to hold the loose threads in!
Here's just a bit of a sneak preview of what I'm making here - it's a smallish purse. I sewed the bottom edges together without turning them inside, in the hopes that I'd get a bit of a frayed edge to echo the appliques. It will still be getting a shoulder strap, hemmed top, and probably a tab and button closure, in the same flannel plaid.