First you need to clean the threads and seat very well in the head.
Next install the bulb and screw the jam fitting into the head.
Now tighten with a flare nut wrench. Do not use a regular wrench you will round off the brass nut corners.
You may have to take off the battery box to get good access. A flare nut crow's foot works but is a big expense for one job when you can just remove the battery box.
Now for the bad news.........
Many have complained they can not get the bulb tight enough to stop leaks even following the above instructions. For some it was they got the wrong gauge (bulb length) and it was hitting inside the head before it got tight.
For some they could get it tight but not stop the small leak.
Was this caused from pitting in the head seat; wrong angle cut on the aftermarket gauge bulb?
I really do not know as I have never had this problem.
As a cure some have reported using a o-ring; Teflon tape; or a nylon washer to compensate for differences in the bulb seat and the head seat.