"Blow-off" valves (BOVs) are so-called because their purpose is to blow off pressure from the duct that connects the supercharger to the engine. When downshifting, the driver lifts his foot from the accelerator, causing the throttle blades to slam shut and all of the air in the intake tube to bounce back against the supercharger blades. This results in a phenomena called "compressor surge," which can seriously damage the supercharger and engine alike. Individual installation procedures will vary.
Find a suitable location in your intake tubing between the supercharger and throttle body to mount the BOV. BOVs don't need to be oriented in any specific direction to work, so place it in whatever position you need to to keep it away from heat and moving components, and make sure to leave three to four inches above it for future spring replacement and tuning.
Mark the location of your BOV's tubular adaptor with a permanent marker. Drill into the centre of your marked circle to create a pilot hole for your hole bit. Cut the hole with your hole-saw bit. You may wish to liberally coat the hole-saw bit with wax or grease for a cleaner cut and less heat-induced metal warpage.
Clean the edges of your hole with sandpaper, and remove any grease or wax lubricant with solvent or detergent.
Slide the BOV adaptor into the hole and hold it very still while you give it a quick tack with the welder to hold it in place. Turn your welder to the lowest workable setting and finish welding around the perimeter of the adaptor.
Insert the O-ring seal on top of your adaptor, then install the BOV itself using the supplied clamp.
Connect the BOV's vacuum/boost nipple to a dedicated vacuum/boost port on your throttle body (one on the outside of the throttle blade, not on the intake plenum side). Throttle bodies typically have several such vacuum ports, but you may need to install a T-fitting in line if all of yours are occupied.
Start the car and have an assistant rev the motor, periodically releasing the throttle to activate the BOV. The BOV should emit an audible "whoosh" as air escapes.
Spray or dribble soapy water around the weld as your assistant revs the motor. If you see bubble then the weld is leaking. You can either remove the BOV and reweld to seal it, or you can "caulk" around the weld with a thin coat of 2-part epoxy. Just make sure to press the epoxy hard into your weld to fill any small pinholes.
Bear in mind that this type of BOV won't work on mass-airflow (MAF) sensor equipped engines. For MAF applications, you'll need to install a recirculating-type BOV that won't drive the computer into fits when it opens.