The spider requires five bills: four for the legs, and one to wrap the body.
(This is the first one of these I've put up with this type of image. I used a Casio QV-10A digital camera to get the images. I've tried to reduce the resolution of the JPEGs to the very minimum required to minimize your download time, but it is still quite graphics-heavy.)
Start by rolling up four of the bills into tight tubes (as explained here), then fold each one in half end-to-end.
Then fold the remaining bill exactly in half lengthwise three times.
Now you have the pieces.
Take each of the four leg tubes and drape them over the flat folded bill near the end. As shown, leave a little bit hanging out the end, as seen to the left in the picture.
This is a close-up of the four legs draped over the flat folded bill. While holding the leg ends tightly together, tightly wrap the flat bill around the whole thing one and a half times.
This picture shows the flat bill wrapping over and trapping the end poking out from the left on the picture above. Make sure that this wrap is very tight, that the legs are squeezed tightly together, or you won't be able to make the body stay together (described later).
This is a pretty good, tight wrap. If it doesn't look like this, don't continue. Work on it until it is solidly tight.
Making sure to hold the wrap tightly closed, turn the whole thing over in your hands. While squeezing the side tightly together, kink each of the legs outward, four on each side.
(Try forcing the bend to occur over your thumbnail, to prevent pulling the wrapped bill out of shape, or loosening the wrap.)
Fold a ninety degree turn into the flap over the end, and then fold the remainder down between the split legs. (Just look at the picture.)
After going between the legs (top of this picture), wrap the flap around the other end and around under.
Okay, this is a rotten picture, but... Take the remainder of the tab (bottom of previous picture, middle-to-right this picture), and tuck it between the legs and the outer wrap on the side that the outer wrap is only one deep.
(I usually use the end of a key to help force this bit down into the body. This has to be tight, or the whole thing will fall apart. If the end is too short, or it won't stay shoved down in there, you need to go back and tighten up the whole body wrap.)
If all has gone well up to now, the body will hold itself together based on the previous step. For each of the eight legs, bend back toward the body.
That's about it. Here's the finished product:
There are two keys to having it look nice and stay together: