low side is fire wall to compressor, High side is compressor to condenser in front of the radiator. On the SWP123 the back is the low side, the middle is the high side. you can also look for a D for discharge (high side) and a S for suction (low side) stamped on the case of the compressor.
the hookup to the gauges is you remove the brass screw on caps on the compressor and then you hook up the hoses, they are old style screw on, not the R134 push on. Would need to see your gauges to know what you have. The assumption is you have no R-12 currently as bleeding it into open are is not legal. So assuming its all leaked out you will have no pressure, so after attaching the hoses (gauges should read 0 psi both high side and low side). you hook up the vacuum pump, open both the gauge valves, turn on the pump and you should immediately see a vacuum. the high side does not read neg pressure so just keep watching the low side. You should get to -29 if you have a good pump and no leaks. Once there close off both gauge valves, turn of the pump, and watch the low side. If it holds you may luck out. if it does not you will need to replace the pump with compressed air or better still nitrogen. open up the valve from the nitrogen then crack open the low side valve on the gauge, assuming you started at zero just allow the pressure to build to about 70psi. Don't go higher, no need for testing for leaks, get some soapy water and spray around the hoses, compressor, drier, any fittings and esp the front of the condenser. look for bubbles to form. Often you will also see oil stains, esp on the condenser that is also a sign of a leak.