Self centering dowel jigs are loved by woodworkers everywhere and make doweling far easier than it would be otherwise.  The main part of the jig is a steel block with holes for the drill bit - this helps to guide the drill bit and keep it from going anywhere it shouldn’t. The holes are located exactly in the center of the block and are perpendicular. The block also has a screw running through it, and turning this screw moves each side of the jig the same amount This is where the “self centering” part comes in.  As you clamp the jig to the wood, the holes on the jib are centered on the wood automatically. In most cases, all you’ll need to do here is mark the dowel location carefully on the wood, including center lines, and then make sure the jig is lined up properly with the center lines. Then, just start drilling.  Depending on the project you’re working on, you might need to make sure the holes are drilled to a specific depth, and if so, then you can use a stop collar to stop the drill from going too far.