Dane, I used a V-splice with the siyah coming to a point. The siyah was taller than the core, I tapered it doen to the thickness of the core only at the tip. I overlaid the sinew over this joint. Also, since my horn strips weren't long enough to cover the joint on the belly side, I used a thin piece of walnut to bolster this area, and I wrapped sinew around the end of the horn on both ends. My horn was also too short to overlay the handle, so I uised a piece of osage as my spacer there. I used a piece of walnut on the handle on the back side, and went over that with sinew before wrapping the "fade" areas with sinew. I ended up covering the sinew with silk and painting that. I used black walnut for siyahs because it is lightweight, and I oriented the grain so that the grain runs parallel to the length of the siyah, not the bow. This lets the string pull against the walnut without it plitting into a particular ring. The string is oriented diagonal to the grain this way, I thought that was safer.
How much of each (sinew, horn and core) is the 64000 question. I used horn strips taht were maybe 1/8", I used a bamboo core that was something like 1/4", and I used a generous layer of sinew that probably dried down to about 1/8". This was for a 50" total length bow with little limb reflex and maybe 30 degree angles on the siyahs. It came out quite heavy, probably in the 80lb range. So I worked the horn belly some, it works well with a scraper. I beveled the belly too, and worked the sides a bit as well. Its still a bit stiffer than I like pulling, I measured it at 70lb but it may be a somewhat less than that now. Next time I'll use a slightly skinnier core. Someone else on ATARN made one about the time I was working on this, and it came out too light. I didn't want that so I intentionally erred on the heavy side. Keep in mind that longer means less hard bend and therefore thicker limb for a given weight. And wider means heavier for a given thickness. I "winged it" on this, but I have a little experience and did some reading first. You might want to copy a bow someone made or an artifact when you make one.