Before you jump into bow making, please understand that it is complicated. There is a vast difference between a quick bow that lasts a week or two and one that will last until the wood rots. There might be times when you need to manufacture something on the fly, but it is always nice to build a lasting bow. You will learn about both here.
Here are a few terms related to bow construction that you should know:
The limbs are the portion of the bow that bends, and the riser is static in the hand unless you are building a bow that is purposely meant to bend in the handle area. This works well with green sapling bows for quick use but also adds dramatically to hand shock.
Whichever bow you are building, it is critical that construction never compromises the outermost growth ring on the back of the limbs. This ring will give the bow the greatest stability when it is bent on draw and will prevent the wood from splitting. For that reason, choosing the right wood is just as important as proper bow construction. Select a hardwood with a lot of fibers in the grain of the wood, which will add to the durability. Hickory, maple, and red oak are the most common woods used to make a bow. Find a piece of wood that is clear of knots and limbs. Use a sapling that is about 3" in diameter and 6' long. Mark the center of this piece with a knife cut right in the bark. Wrap your hand around the bow with the top of your fist positioned at that mark and then place another mark at the top of your grip.
Now that you have found the center, make sure not to remove any wood from this area. Examine the piece and see if there is already a natural bend in the wood that you can take advantage of. When finished, the profile of your limbs should be almost triangular with rounded corners. Sharp corners are prone to cracking.
When looking at the top and bottom of the riser, you will want the limbs to slowly taper from approximately 3" wide to 1" wide at the string nocks. Use your knife or axe to begin removing material in a slow taper from the riser down to the limb tips. Remember, the more wood you remove, the lighter the bow’s draw will be. You want a triangular shape that reduces the length of the limb from riser to the tip. As you go, check the tiller of each limb of the bow. You want the bend to be an even arc. Looking for the curve takes a good deal of practice, and you will probably break a few bows while you are learning. Tiller both limbs so that they bend about 3" in an even arc when you place the instep of your foot on the limb and push on the riser area.
Now it is time to string the bow and begin the final tiller of the bow. Using a string that has ten times the tensile strength of the bow draw you desire is important. For example, if you want a 50-pound bow, you will need a string that has a 500-pound tensile breaking strength. I find that #36 bankline fashioned into two-ply cordage gives about 700 pounds of tensile breaking strength, which is plenty for any bow I would make. When cutting in the string nocks, be careful to make them only as deep as absolutely necessary and angle them at about 45° down toward the riser.
To connect the bowstring and string the bow, use a solid loop on one end of the string like a bowline and an adjustable knot like a timer hitch on the other that can be secured with a half hitch. Step through the bow and bring the string up and around the limb tip at the top with the loop already seated in the lower limb nock. Brace the riser area against the back of the knee and bend the limbs equally. Do not try to get the bow to final bracing height. The actual distance of the string from the belly of the riser is usually about 6"–7" at this point. Just string it tight enough for the bow to bend about 3". After this, you can begin to exercise the limbs by slowly drawing the bow with no arrow, a few inches at a time, yet never to fully drawn yet. At this point you are just breaking in the limbs.
The best way I have found to examine the tiller while in the field is to sit on the ground and put the riser of the bow under the instep of your foot. Pull the bow with both hands and look over both limbs as they bend. If the tiller is off and one limb is bending more than the other, you will need to remove small amounts of material from the belly of the stiffer limb. Take care not to overdo it because you will end up with a bow that is too lightweight. After the bow has been stretched about 50–100 times and the tiller appears to be good, you should tighten the string to a brace height of about 5". Start the process of flexing and checking the tiller again. If the bow is too stiff again, remove more material but be careful. If it is good and you complete the exercise process, then you can go to full brace height or 6"–7". The end goal is for the draw to be good enough to hold at full draw until the count of ten. Make sure the tiller is even on both ends.
Remember that at this point you have a green-wood bow and it will take on a permanent bend very easily, so take off the strings when you are not using it. Let the wood dry naturally, and it will eventually mature into a pretty good bow. As it dries it will get stiffer, so eventually you will need to seal it with oil to prevent further drying and possible cracking.
The wood selection process should be the same for a permanent bow. Generally you want to harvest the wood in the spring. Split the log four ways and then you have four possible bow staves. Remove the bark and keep the wood dry and off the ground for at least four seasons. Once the staves are dry, select the best one that does not have any knots or twists. Once the wood is dry, a draw knife, heavy horse hoof rasp, and a shaving horse might come in handy. When working with dry wood, you might actually want to carve a riser and shelf right into the wood. Then carve a flat side into the arrow side of the riser with a small shelf for the arrow—about 1⁄2" for the riser and 1⁄4" for the shelf is plenty.
Since the wood is dry, it needs to be treated with great care during this process so that it does not crack. Seal it immediately with animal fat or tallow. Again, never store a wood bow in the strung position; always unstring and lay horizontally across two pegs or forks. Avoid excess heat or direct sun on the bow.