There are lots of threads about this with lots of different answers. To date I have been through trying to make a jig myself, to a HF self centering jig but moved on to a Dowel It. One thing I have learned is to make sure your limbs are parallel. The next thing is on using the jig; no matter how good the jig there will always be a bit of play. So when I put the jig on the limbs or the riser, I will snug it down, then I will put my thumb in the middle of the jig (the part with the drill guides) pushing it against the surface to be drilled. Then I will loosen the jig and re-snug it down a couple of times. That seems to even out the play and do a very good job getting my holes centered and inline.
I really like the Dowel It (2500 I think it is) because all four holes are tapped for drill guides and I can move them around how ever I want.