I’m assuming you have the check valves/needles removed.

If you’re only dealing with the main body, there isn’t much to it.
Two holes at 90* to each other, connected at the intersection.

I’d just use a small drill bit and spin it in with my fingers from both sides, or a stiff piece of wire, like a paper clip.

If you use a drill bit, be sure it’s smaller than the ID of the seat for the check valve so you don’t damage it.