You will need a guard from a computer fan. They are about 3 inches/7.5cm wide.
Step 1: Marbles
You will need lots of marbles, a hot glue gun and some string to hang the decoration.
These marbles are 'craft' marbles from a discount store. Any marbles that let the light through would be fine.
Work out a pattern you're happy with. Then, using a hot glue gun, put two dabs of hot glue onto two guard tracks. Quickly place a marble on the glue and hold for a few seconds. Continue to apply marbles until you've got a piece of beautiful glass art.
Update : tulekah kindly suggested putting marbles on both sides of the fan guard so that it looks good from front or back.
Step 2: Fewer marbles
Step 3: Fried Marbles
The link has instructions on how to fry the marbles from a person who has tried every method and found that baking them in the oven worked best.
Step 4: Beautiful just as they are
Step 5: Beads
Step 6: Tie on the first lot of beads
Put a bead on the thread. Check that it is sitting nicely then put the rest of the beads on for the first segment.
Step 7: Take it slowly
Just put one bead on and check that it sits properly. As you can see I have taken the thread around a bar that is on the outside of the row where the beads will go.
This helps to keep the beads stable where you want them to be. If you get yourself into tangles trying to follow the loops in the photo, never mind. Just learn as you go along and you will find the best way of threading for yourself.
The beads have to sit in a curve to follow the curve of the metal bars. If you pull the thread too tight they'll form into a straight line and look out of place. We'll be correcting any sag that comes about from loose threading in Step 10.
Step 8: Progress
In the 1st row you can see that I pulled the thread a little too tightly. Now the yellow beads are in a straight line instead of a gentle curve.
Step 11: Tucking everything into place
The bottom photo is what it looks like from the back. The thread is more noticeable from the back because it's not going between the beads.
At any point of the bead project you only need to follow your own common sense. If you are not able to get good results from looking at the photos, it doesn't matter. Just do what I did: take note of what works and doesn't work and do what does work!