Take a bunch of fibers. Add as many as you need to make it the thickness you want.
Fold the fibers. Do not fold them exactly in half. Leave one side longer than the other. That will become important when you are trying to extend the fibers.
Hold the fibers at the place where you have folded them, as close to the edge as possible.
Take the top strand and twist it away from your body.
While still holding the top strand, grab the bottom strand and twist the two strands together towards your body, moving the top strand over the bottom one, so the bottom strand now becomes the top one.
Now twist the new top strand away from your body, just like before.
Again, while holding the top strand, grab the bottom one, and twist both strands towards your body, making the bottom strand the top one.
Keep repeating the process until you get the length of rope you need.
Another method for doing the same thing, which depending on the fiber may go faster, is to place both strands on a flat surface (usually you thigh).
While holding the location where the finished rope ends with your left hand, roll both strands away from your body.
When you release your left hand, the ropes will twist around each other. Grab the new place where the fineshed rope ends, and repeat.
Eventually, however, your fibers will run out. Take a new set of fibers, and place them along side the fibers that are running out.
Overlap the new fibers with the old ones.
Twist the new and old fibers together, away from your body.
Then continue using the method above, to make more rope.
The longer the area which you have overlapped, the stronger the connection will be. This is why it was important that the fibers not be folded in half. This way you are adding fibers to only one strand at any particular location.
Repeat the process as many times as necessary.