missjane - I'm happy to share what I know on the fusible webbing! You Advent calendar sounds like a great idea! Are you going to be using felt? If you are, I would definitely advise using wool felt or a blend - it will hold up to being held and used so much better than acrylic felts.
I'm lucky enough to have a craft store nearby that sells it in small sheets as well as the big bolts - it isn't a chain store - I haven't seen any of those carry wool felt
Just a thought!
In any case, the fusible webbing itself is called just that or sometimes it is called "Wonder Under"
It is the in the same section as interfacing is at the sewing store. I had to ask the ladies the first time to help me find it. They cut it off the bolt just like they would fabric, in whatever amount you want. It will be paper on one side and slightly sticky rough on the other. And layered with the webbing is a very thin paper set of instructions that repeat over and over throughout the whole bolt. So make sure you get a complete set of instructions with what they cut off for you. If you get at least a half yard you should be fine - but check!
The instructions are pretty clear, but I'll summarize what I do below too:
FUSIBLE WEBBING TECHNIQUE
1.) The easiest way I've found to do this process is to place your template under the fusible webbing and trace it. BUT remember to do the reverse image. You will be using your pen, pencil, whatever to draw on the side of the webbing that feels/looks like paper. (The other side will feel gummy and webby)
So if you are tracing an image you've printed, you either have to flip the image before you print it or turn the paper over and trace all the lines onto the backside of the paper first.
I've also simply cut all my pattern pieces out of the computer paper, flipped them over, and carefully traced around them. Sometimes I have to print multiple copies of the image to get full pattern pieces of every shape I'm trying to create.
2.) Once you have traced everything onto the paper side of the fusible webbing, roughly cut out your picture--not on the lines. I try to leave a small margin outside the lines because the more extra you leave, the more of your felt/fabric you waste.
3.) Iron the cut out shapes onto your desired piece of felt or fabric. The instructions that come with the webbing should cover this part, but basically you place the shape with the sticky/webby side down. You cover it with a dry cloth - flour sack towel or something like that is best, you want it lint free and not too thick. And then you press down with the iron for several seconds. You want to get the shape good and stuck to the fabric but it doesn't take too long. Note: I use the wool or cotton setting on the iron.
4.) Once it has cooled a bit, you can then cut out your shape on the lines.
5.) Finally, you peel off the paper backing which reveals another sticky surface. You place the shape, sticky side down on desired felt or fabric. You use a damp cloth - usually the one I used during the dry pressing phase) to cover the image. Press down with the iron for a good 20 seconds or so - it will hiss and steam that's normal.
Carefully lift up iron, check image - then I usually press again with damp cloth just to make sure the shape is stuck on good and tight.
That is it! I started with basic shapes for quiet book pages and once I gained confidence I've found I can do much more complex things, layer images, etc.
I used this technique on the felt ornaments I made recently that look like little girls in winter coats. It is so helpful to stick down the felt pieces with this webbing before sewing, embroidering, etc. They don't move around and distort that way.
Happy crafting! I'm happy to discuss it more if you have any questions
