I've been asked by lots of people to write instructions for the chain I used for the strap of my 2 Color Geometric Amulet Bag Necklace. It's a right angle weave variation that can be done with either 1 or 2 needles.

You will need seed beads, I used size 11 seed beads on that necklace.
Needle or needles
and thread. Something that will do will multiple passes through your beads. If you are using Japanese size 11 seed beads, most threads are fine. I do recommend something strong if it's weight bearing.
The current step in the diagram is shown in pink, the previous steps are greyed out.
Single Needle-
String a stop bead, and then string on 4 beads, go back through the first, second and third bead.

Add 3 beads, and go through the bead your thread came out of on the previous step, and the next 2 beads.

Add 11 beads, go through the bead your thread came out of on the previous step to make a 12 bead loop.

Now it's time to make that diamond shape pop out. Go through the next 2 beads, skip the 3rd bead, go through the next 2 beads, skip the 3rd, go through the next 2 beads, skip the third, go through the next 2 beads, skip the third, then go through the first 2 sets of 2 beads you started this step with, and through the bead you skipped.

Continue making the chain this way. 2 4 bead right angle weave units, and one 12 bead diamond unit.
Double Needle-
Depending what your attaching this to, you can either use one long strand with needles on both ends, or you can use two separate threads. If you want to use one long strand, the first bead goes roughly in the middle. It's called either 2 needle right angle weave, or cross weaving because of the way the two thread paths cross.
The thread paths are shown in 2 colors. I'll call them right and left, but they do switch sides, just remember the right thread is the same thread throughout and so is the left.
From the start bead, add 2 beads on the left thread, and one on the right, pass the needle from the right thread through the last bead on the on the left thread in the opposite direction of the the left thread's path. Add 2 beads on the right thread then one bead on the left thread, then pass the left thread through the last bead on the right thread in the opposite direction of that thread's path. Add 6 beads on the left thread, and 5 beads on the right, pass the right thread through the last bead on the left thread in the opposite direction of the left thread's path.

With the left thread, go through the first 2 beads, skip the third, go through the first 2 beads, skip the third, next 2 beads, skip the third, next two beads, skip the third, then through the 8 beads you just went through (make another circle through them) and then pass your needle and thread through the last bead, coming out of the same bead you started in with your needles and threads in opposite directions coming out of that bead.

Continue in that fashion until your chain is as long as you want it.
Just for fun, I made a background and page border that will tile with the images I used for this tutorial.

If you'd like to learn how to do a lot of types of chains, I really recommend these two books.