I've made "window quilts" in the past, and you'll find lots of instructions on the internet. I was able to get new packing quilts (used to cover furniture/appliances, etc. during moving) for $5 each for this project, but old comforters, rugs, and other things can also be used.
It's more than just thickness/layers of materials, it's also about trapping the air on all side, between the window and the "quilt". Most window quilts are connected at the sides and bottom of the window with velcro, or you can also use strips of plastic magnets (which have a self-adhesive tape on one side) found at craft stores (avoid metal because condensation will make it rust).
Another method I've done is to make the window quilt with 2 sleeves (the place where you put the curtain rod) - one at the top and the bottom and put tension rods in both to secure the window quilt snuggly at both the top and the bottom of the window, rather than hanging from a rod, which will create that chimney-like air movement from the bottom of the curtain to the top (which isn't energy efficient). If you need to open this window quilt, just move the bottom tension rod to the top of the window.
It's also important that warm air rising from the room doesn't travel from the bottom opening of the window quilt through the opening in the top. You need the window quilt to trap air in, not let air travel through it. You may also need some sort of pelmet, cornice or valence at the top to trap air, depending on the style of curtain you use.
I used the Mylar Emergency Blankets as the outside (back) layer because they are cheap and water resistant, which is important due to condensation from the windows. If you just have fabric, it can get water stains on it. I've also use inexpensive flannel-backed plastic tablecloths (plastic to the outside).
Here's another good idea to create a barrier that is non-invasive and cheap (great for renters). This is a new one to me and we're going to do this to as many windows as possible this year. You can cut bubblewrap to fit windows and apply it to the window by spritzing water on the glass and putting the bubble wrap on the window. I'd suggest using distilled water since minerals in tap water can actually etch glass and may mar your window. You'll still have some visibility and light, but one more layer of insulation to the windows. Just peel off when winter is over and use again next winter...
This one is a good glue-gun project.... I made a 2-piece shutter that fit into the basement egress window out of several layers of cardboard (got some large appliance boxes) glued together - to fit the window. I wrapped the whole thing in aluminum foil (to help make it somewhat fire- and vapor-proof). You can then finish it according to your budget and brilliance. Make sure the back is covered by something that is vapor-proof (that dreaded condensation again), and fancy up the front with whatever you like (decoupage, or glue strips of tissue paper on it to make it resemble a stained-glass window, use spray adhesive and attach some cloth, wrap the front in a poster, rip up brown paper bags and glue (plain old white glue) the pieces on the front, etc.....) This type of shutter can be quickly put in place each evening and removed during the day.
-Gingerbread
