Not many quilters are familiar with fusible thread and I have not seen very many ways in which it has been put to use. I use it for machine applique. It looks a lot like dental floss. If you lay out a line of it and put your iron near it and steam it, the thread will shrivel up. It will bond fabric to fabric in a very thin line - the width of the thread. If you use a zigzag stitch, it gives more of a bonding area but for my purposes I only use a straight stitch because I want the least amount of bonding possible.
Although you can use a fusible thread for your upper thread, I mainly use Charlotte's Fusible Web in the bobbin - wind it slowly and evenly onto the bobbin. Use MonoPoly monofilament thread in the top. Some of my students are concerned that the monofilament thread might melt. I have never had it happen because I use a polyester monofilament, MonoPoly. Do NOT use a nylon monofilament or a polyamide (another fancy name for nylon) monofilament because it will melt and get all over your iron. The invisible thread only holds the fusible thread in place until the fusing has been done. After that, the monofilament becomes redundant. With Charlotte's Fusible Web you can even pull the line of MonoPoly thread out after fusing, while still warm, because the monofilament slides through the nylon fusible thread. If you prefer you can use a cotton machine embroidery thread instead of monofilament but you will need to change the thread often to match your fabric and you cannot pull it out.
Stitch through one layer of fabric only. You have lost the purpose of using the fusible thread if you stitch 2 different fabrics together. You may need to adjust your machine tension to get a stitching line that lies flat. Do not let the upper thread pull the fusible thread up to the surface of the right side - this is rather unlikely to happen because the fusible thread is so much heavier than the MonoPoly . If the bobbin (fusible) thread is pulling only slightly and making the fabric cup, you can clip the fusible thread every once in a while because the small gaps will not interfere in the process. Clipping will release excess tension and the applique will lie flat. Do not backstitch when you begin or end stitching - it is not needed. If you are stitching short lines, pinch the fabric where the stitching stops before pulling it out from under the presser foot to avoid pulling out all the stitches. Make sure there are no loose stitches at the beginning and end of stitching by pulling the tail ends of the monofilament and then trim the tails from the fusible thread and then from the monofilament.
You can now fuse the stitched fabric where you want it. When fusing, be quick and firm with your iron for 10 seconds. Don't hover over the work with the iron where the steam may make it shrivel and pucker the work. I would actually rather use a dry iron than use steam but even just the heat from a dry iron may cause the fusible thread to shrink if you don't apply pressure immediately. Note: this is only a temporary bond. The thread bond by itself will not withstand much wear and tear. It will need to be permanently finished after fusing is completed.
Because fabric appliques are only fused around the outside edges of the shape, you can cut away the background fabric from behind the appliques if desired. I assemble complicated applique pieces independent of the background by only fusing the shapes together along the lines they have in common and then stitch around the outside of the constructed applique and fuse it in place on the background. A lot of trimming is used in this process.
Once a project is temporarily assembled or constructed with Charlotte's Fusible Web, it needs to be finished. Put a stabilizer on the under side of the project. On the right side of the project, satin stitch or thread paint using the appropriate color machine embroidery thread over the raw (fused) edges of the shapes. Remove the stabilizer and use the completed piece as desired.