@Brayden Brehaut First post? Welcome to the forum from Dayton, Ohio - birthplace of aviation (and the radar detector).
You could put the Tee several places but a good location is in the vacuum line leading to the blow-off valve. I've attached a couple of pics. The Tee I'm using is brass and leads to a Defi sender. This was Defi's recommended Tee placement. The sender itself is a small black plastic gizmo mounted on the strut tower - impossible to photograph and almost impossible to reach ;-) The BOV in use is made by GFB (thanks, mate!)
FWIW, use good quality hose that make a tight fit to the Tee. Most vacuum lines are 'suck only', which tends to make the hose joints pull tighter. This particular vacuum line occasionally experiences pressure - 10, 12, 15 psi or more, depending on your mods, tune (and madness). I'm no fan of plastic Tees, either. Make mine metal, please.
In the following picture, the above Tee and hoses are on the right side of the picture - just left of a line drawn between the red Grimmspeed AOS and the white brake fluid reservoir.
Here are the results of me installing a Savage Performance Mechanical Boost Gauge.
It idles around minus 20 and then Runs to around 13 to 14 PSI full Boost
Also a photo of the placement of the gauge in the car.