Daven Thrice writes:

I've scrapped enough metal to set my ego aside and admit that I need
help with the very basics of basics. How do I make the measurements,
mark them, and then drill the holes?

Straightedge and carbide scriber for the lines. ($20)

Digital calipers' points as compass points to cross-scribe the intervals.
($20)

Spring-loaded center punch to dimple the points to drill. ($3)

Let the dimples center the drill bit.

Should be accurate to a few thou or 0.01" depending on your skills.

A loupe helps.