Basically you hook the transserve machine up to the trans cooler lines, fill the machine with however many litres it takes and it will flush the old oil out, flush the cooler and torque converter at the same time. The level will have to be correct to start with though as it only replaces what you put in eg 6 litres in, 6 litres out.
Check the level with the engine running in P or N. The pump will fill the converter.
Dropping the pan (to replace filter etc) and depending on transmission size will only give you about 3- 3.5 litres out of 7,8,9 litre systems
Can you get a trans model code? if so i can find out at work how much oil it will take.