Turn off your Pro Tools audio interface and the VT-737SP, if they are on. It is generally safer to connect and disconnect cables when electronics are switched off.
Connect the "line out" end of a link cable to the outlet labelled "OUTPUT LINE" on the back of the VT-737SP.
Connect the "line in" end of the link cable to one of the "Line Input" slots in the back of your audio interface. On some Pro Tools--compatible audio interfaces, the "mic" input may double as a "line input." This will be indicated somehow by the labelling. Using a "Line Input" slot will bypass the audio interface's pre-amps (if any) and replace them with the VT-737SP's input.
Connect your microphone or instrument to the correct input on the VT-737SP. The microphone input is on the back of the unit and is labelled as such. The instrument input is on the front and is also labelled.
Turn on the VT-737SP. Because the VT-737SP is a vacuum-tube pre-amp, it is good practice to allow it to warm up for about 15 minutes before use. However, this is not necessary, and it will not harm the VT-737SP to begin using it immediately.
Turn on your Pro Tools audio interface, and ensure that it is connected to your computer. This is usually done with a USB or FireWire cable.
Open Pro Tools. You will now be able to access the VT-737SP in Pro Tools. It will function in the same way a pre-amp from your audio interface functions within Pro Tools.