Step 1: Materials:
Roll of 2" wide tape (I used aluminum tape)
Bag of Grommets and crimper
Bag of curtain hangers
*I don't know about in the US - I've had these before in my camp pack as "emergency blankets" for protecting you in the event of shock or if you have to sleep somewhere in the cold. The emergency blankets don't have any backing on them, so maybe you might want to glue some foam/paper/cloth to the back. Super 77 tack is perfect for this.
I'm currently in Japan, and in the 100Yen shops (great places for makers - come to Japan just to stock up for supplies) they have these mats that are meant to be used as picnic mats.
Step 2: Constructing the curtain
Then decide where the curtain will fold and crimp a small grommet into the fold as shown. The grommet should be able to punch thru this - I've crimped grommets thru fabric and it usually cuts a little hole for you. If not, an X-acto blade should clean it up nicely.
- small tip, I made it so that the folds end up rubbing against the window - this makes it slightly annoying to push open the curtain completely; two hands are needed. See if you can fix my mistake on your design. (-:
Step 3: Add extra lengths if necessary
I taped it in the front using aluminum tape (doesn't make much of a difference, but it looked better on the shiny side) and used normal packing tape to secure the back.
Step 4: Tips and suggestions
You can make two curtains, one for each side, but I found that this one curtain is easily hideable behind one of the decorative curtains.
Whole project cost:
$4 for blankets
$1 for grommets
$1 for tape
I think this will save a lot off of our summer electricity bill since I can't feel the heat from this window anymore. The wall next to it heating up is another issue, but I don't see how to really fix that.