There should be 2 small holes in the top of the shaft in front of the hand grip where a V clip is to be mounted if there is not one on there, The meter will slide into that clip and snap in place if slide in far enough.
There is no instructions for the meter, but easy to calibrate. Once installed you turn on the GT and put the sensitivity to auto or low so it will be stable and not falsing. The meter has to be set while in disc and when you swing a new quarter across the coil back and forth look at the numbers and try to keep it the numbers as stable and as high as you can, if less then 180 you turn up the calibration on the back, if more than 180 you turn it down until you get 180. It can be touchy to get it to 180 and a 179-180 will work too and I try not go higher than the 180 as i know where my good number will be at 179-180 on copper and silver, 176-177 on zincs and IH pennies, and 143-145 on most nickles for me. If you ever see a number higher than 180 while in disc it means the calibration is off as nothing will read higher than 180.