However, before you can make your own bow, you need a bowstring. Making Bowstring is difficult enough to require a seperate thread, and some of the concepts can be hard to explain. It is definatley one of those things better explaind in person, But I'll try and explain it with Illustrative Photo's to help.
So, lets get started:

Tools List: Hammer, Nails, Sharp Knife, Tape Measure, Wax (prefarably bees, but tallow will do). Of course, String Thread. I'm gonna use Dacron B500, but you can use ordinary thread. However it must be plain untreated thread, and you should work out it's breaking weight. I'll explain this more later. Oh and you need a Fence

well, actually, and straight plank of wood you can hammer nails into will do, I just use the fence 'cos it's at a useful hight.
You now need to hammer in Nails, the distance between the nails will be the distance of the string. The Distance should be the Length of your bow, plus another 20". I'm 6', so my bow is 6', which makes my string is

You also need to hammer in a third nail, this one 12" from one of other nails.
Now. we're gonna wind out the threads onto the nails, that will make our string. This string is for making a bow, so it needs to be extra strong and tough. I'll be making it at 50lbs (What I'd expect an adult male to draw after a little practice, and what I can draw comfortably for a few minutes). Now, This is where we need to know the breaking point of your thread. Your bowstring needs to have a breaking point of 6 times the draw weight of the bow, so the string needs to have a breaking point of 300lb. So if a single strand of thread, has a breaking weight of 5lb, we'll need 60 strands of it.
In this case, and because it's a string for making a bow, We'll be making it for really really rugged, means we'll use 18 strand, instead of my usual 12 strand. Dacron B 500 has a stretching point of aprox 25lbs.
So, what we're gonna do, is make 3 skeins of threads, each 1/3 the total number of threads required, so thats 3 skeins of 6.
So, tie one end of the spool to the nail

And unroll the spoll a way, and around the other nail 92" away, and bring it back, 3 times

Then cut the threads at either end, giving you 6 identical strands 92" long. One Skien

Now, do it again, and again. Giving you 3 skeins, of 6 threads each, 92" long.
Now, there is one part of the bow string that suffers most, where it meets the top limb in a loop, it is particularly prone to wear. So we are going to thicken the ends of our threads, which is why you have the nail only 12" apart.
Make 3 skeins of 6 threads only 12" long, and this is what you got

3 long, 3 short. If you are using ordinary threads, now you must wax them. warming the wax in your hands, and rubbing up and down the length of the thread from centre out woods, impregnate the thread with wax. If you are using the Dacron, or another modern bowstring material, don't worry, it's already been done.

Now comes a tricky bit. tapering the threads. With both your short and your long threads, they need to have a distinct taper so they will marry together well later. the best way to do this is by taking 2 threads from a skien, and placing one end of one, 1" below the end of the other.

Then, run the thread through your fingers, to ensure it sticks together and preseves the taper, the wax will hold it together.

Then you take another thread from the same skein, and put it another inch below, continuing the taper. Then run it through your fingers again and take another thread, and so on...

Till you have 6 taperd strands, 3 long, 3 short.
Now, take one long, and one short, and pinch them together, between finger and thumb you can hold them tightly in one hand, and marry the treads into each other with the other hand, starting like this

And working from the centre of the taper to one end, then from the centre of the taper to the other end. Giving you this

A solid thickend end of a single strand 92" long.
Do this to all the others, so you have only 3 long strands, thickend at one end. Then roll them up till you have about 18" left, sit comfortably, and put them parrallel on the ground infront of you, with the rolled end to your right.

Then take the three strands firmly in your left hand, pinch them about 9" down from the end.

Now comes the fun bit, and the bit hardest to explain.
Using your right hand, grab the strand that seems furthest from you, twist it hard away from you using your thumb and forefinger. then bring it across the other strands, sip it under your left thumb and hold it tight. To avoid getting a tangle at your rolled ends, bring your rolled end across as well, towards your self.
The take the next strand, which should now be furthest from you, and do the same twist it hard away from you using your thumb and forefinger. then bring it across the other strands, sip it under your left thumb and hold it tight. Keep doing this, and rearranging your skeins, but resist the temptation to open your left thumb, keep it tightly bound.
After quite a while of this twisting, you can give in, and look at whats in your left hand

Nice, eh? a short bit of rope. But keep going, this short rope needs to be 4 or more inchs long.
Once you have this length of rope though. you form it into a loop

And pinch the free ends together. because at each end of the ripe there will be 3 strands, marry one short strand, to one long strand, as you can see in my picture.
Then you should just have 3 strands, with a loop.
Now, you begin twisting again. take the three strands firmly in your left hand, pinch them, then grab the strand that seems furthest from you, twist it hard away from you using your thumb and forefinger. then bring it across the other strands, sip it under your left thumb and hold it tight. Keep going till you've run out of taperd length, and have reach the plain 16 strand section.

Here endith part you