Measure the tap...and drill a hole big enough so you can start the thread.

Then measure the fitting you want to screw in and reference the widest point to the same portion of the tap.

And just make sure you dont screw the tap in any further than that...ideally slightly less to ensure the fitting secures tightly into the hole.

Just hand cut with some sort of lubricant, oil, cutting lube etc. But go slow and remove and test fit the fitting if need be to ensure you dont cut too deep