There are a few good threads already posted for juniper.....but I will attempt to post something original on the subject. My experience with juniper has been with Ashe Juniper (Juniperus ashei) that is common here in central Texas.
Ashe juniper doesn't like to grow straight. Finding a clear, straight piece is very hard unless you stumble upon a juniper "forest". The more mature trees will yield the best staves, in my opinion. The bark on these mature tress will peel off easier too.
Ashe juniper has clearly defined sapwood and heartwood. The sapwood thickness varies greatly but is thickest on the compression side of a limb/trunk and on vertical limbs/trunks. (I have only worked with sapwood...so that will be my focus in this discussion).
I like to use the thickest piece of sapwood that I can find, peel the bark off immediately after cutting, split, and reduce the stave to near-finished dimensions as soon as possible. Ashe juniper checks badly on the end grain so I seal ALL end grain no matter what size I'm dealing with. I use a hand axe to rough out the stave and a rasp to clean up. This is the easy part....ashe juniper splits easily and responds extremely well to hand tools.
Now comes the hard part....the shaping. If you have a straight juniper stave and want a straight bow, you can skip this part.
Heat bending applied to green or dry ashe juniper works equally well (and equally bad). What I mean is the wood has "memory" and will want to revert back to it's original shape over time but it responds very well to heat and bends fairly easily. I've made several ashe juniper bows and ALL of them need re-shaping after sitting for a while. Some of my bows have better memories than others....but all are a pain in the butt. Also, the thicker the bent area, the better the memory. After the first shaping, I let the stave dry for a least a month indoors. You can wait longer if you wish.....the longer the better. You can also tie it to a form....but I have not tried this so I don't know how well it works. I believe Ishi used a form for his juniper bows...so it's probably a good idea.
OK, once you have a dry stave and begin to tiller, the ashe juniper will begin to show its best attributes....incredible flexibility and strength (for its mass). I should mention at this point that I have not sinewed any of my ashe juniper bows.....for two reasons: One, I never cut through the growth rings on the back of the bow and, two, the wood bends so well that I feel it would be overkill at this point. In the future, I plan to make bows stronger than 40# and shorter than 48". These will probably need to be backed but, for now, unbacked ashe juniper does very well with my standard 40# @ 22'' draw, 48" bow design.
Jim Hamm mentions that he likes to apply sinew BEFORE tillering. If you want to be extra safe, or if you've cut through the growth rings, then this is a good idea.
Tiller the bow like any other, except keep in mind that juniper is a softwood. Your finished bow will be a lot thicker than hardwood bows of the same strength. I've made ashe juniper bows with both wide and narrow limbs with good results. The wider-limbed bows are faster, but they "vibrate" more when shot.
The nocks I use are wrapped, not cut (I've had a couple of cut nocks explode on me). I wrap sinew or thread (set in glue) around the end to create a shoulder for the string. I also wrap the tips to create a "stop" or "bulb" to prevent the string from suddenly sliding off the end of the bow......one of the hazards of short bows.
The only other advice I can give here is to "overbuild" the bow when you start. Ashe juniper becomes stronger as it dries....but not much. When I first started using ashe juniper, all my bows ended up being in the 20-30# range when I was trying for 40-50#'s! Maybe it's because I had just finished a period if time when I was making ipe bows.....I don't know. In any case, once you find a good piece of ashe juniper, BE VERY CAREFUL. You don't want to waste it! The finished bow will be well worth the effort.