NOTE these are 1/4" NPT (National Pipe Thread Taper) which means the hole size you drill needs to be 3/16"
1) Drill a 3/16" hole into the PVC pipe with a Vari-bit. You can check it as you go so you do not make it too big. This is slightly smaller than the standard tap size, but it will make a good tight fitting. Avoid getting too close to fittings.(Fittings means PVC fittings. Do not drill too close to a 90 degree PVC ell or similar fitting.)
2) Using an Allen wrench tap the hole in the PVC fitting with the plug, the plug will get tighter as it screwed in as it is tapered
Remove the plug and insert the combustion analyzer testing cone
Center the probe and lock down the set screw.
When finished testing screw the plug back into the test port. You can use Teflon tape or pipe dope approved for PVC if desired.
Finished job makes a leak free tamper resistant access port ready for years of combustion testing.