The low side of the a/c system will be located on the a/c pipe that goes from the a/c condenser to the a/c evaporator. Sometimes it will be on the a/c accumulator, but it can be anywhere between the accumulator and the evaporator.
Based on other mid/late 90's Audi A/C systems, there should be a pressure sensor on the low side line. Unplug/Unscrew this sensor and screw on a Mastercool MSC82272 adapter. Now fill though the low side the usual way, remembering to jump the two contacts on the sensor's harness to make the compressor kick on. When finished, replace the sensor.