[Superman]

Threadmills need to climb-mill, so for a right hand thread, start in the centre bottom of the hole, and helix into the cut going CCW, and screw out. This avoids re-cutting chips that tend to wear/break the root radius on the cutter.
Through coolant (best) or flood required.
RPM is of lower importance (get it close to OEM specs), but feedrate is. Depending on tool diameter, you may only be doing a 0.080" circle, so start slower.

Gun tap would be the cheapest, fastest, least risky method around for a through hole.
Machine tap for blind holes.
Threadmill if you have the know-how, and tooling

[John Roberts]

Many thanks guys. I know this is not the easiest way of making threads but I really don't want to hand tap 30 holes.
A good point made regarding going in slow for its first cut.
The thread mill has not arrived yet, which is a single point carbide, coated cutter. I guess the cutter will be about 5 mm dia and the narrow shank about 3 mm x 1/2" long.
If I can get it to survive the first few holes, it may complete the job without breaking. One way or another I will learn something useful !!