I would suggest the following approach:

-prepare a block that is the same width as the ash board. The block should be a little shorter in length than your drill bit. The drill bit that would likely work best here is a lipped brad point.

- mark the centerlines of all faces of the block

-use a drill press, clamp the block in place so it doesn't move, and be sure the table of the press is exactly perpendicular to the drill spindle. Drill a hole right through the block. I would say it would be best to orient the hole parallel to the grain of the block (i.e., the drill enters the end grain).

-after the hole is through, check that it emerged dead center. If not, try again, but fussier

-if the hole is where it should be, slice the block across the grain at the same angle the stair takes. This cut needs to be accurate and leave the cut surface flat across.

-attach a couple of flanking pieces to the block, cut to the same angle as the block, which will fit to the sides of the ash board nosing. These serve to auto-locate the block (which is used to guide the drill) on the ash piece. You will need to mark the block in a manner so as to allow it to reference accurately to the location of each hole. Sighting down the hole in the guide block will likely be not accurate enough. Probably making the flanking pieces in a 'U' shape would allow the marking line of the side of the guide block to be referenced against a line marking the center of the hole on the ash piece (a line that is marked right across the width of the ash piece, and thus visible from the side when the jig is placed atop it).

-you will need to devise a way to keep the block exactly fixed in place (so as not to slide up or down or to lift off the surface) while drilling the hole into the ash piece. That means the jig needs to be clamp-able or screw-fixed somehow. If using screws, of course they should not mar any visible finished surface. likely, something could be configured with the flanking pieces of wood on the sides of guide block

-when you drill you will have to go slowly and carefully at first as the edge of the drill bit is contacting the ash surface on the uphill side only and this creates forces on the bit that will want to push it off line. It may be worth using a Forstener for the first 1/8" of drilling depth or so, and then switching back to the brad point. It may pay to apply removable masking tape to the finished surface. before drilling to limit any possible tear-out. Don't drill all the way with the Forstener, unless the depth of hole is quite shallow, as Forsteners are designed to be used within the confines of a drill press and can easily wander laterally when used in a portable electric drill.

Jeez, I guess a picture would be worth a thousand words here eh?