Figure 8 Knots are useful for many things.
You can tie two ropes together using the figure 8 knot.
You can tie the figure 8 on a bight and connect it to something.
It can be used to hold down certain items when driving on the highways.
Rock Climbers use the figure 8 knot very often.
There are other variations of the Figure 8 knot that I will go over another time.
For now check the pictures below for a step by step guide to tieing the figure 8 knot.
First Take the Rope like I have it here. 
Now here wrap it over, and than under. And bring it up towards the loop.
Lastly, take the end and bring it up through the loop at the top.
Now evenly distribute the rope so the knot fits into position properly. 