low pressure port

The high side port is smaller around than the high side. Here is a guide and some diagrams to help you get the job done.

https://www.2carpros.com/articles/air-conditioner-how-to-add-freon

and

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

and

https://www.2carpros.com/articles/how-a-car-air-conditioner-works

The low side port is on the tube or hose that comes from the firewall and goes to the compressor.

Please let us know if you need anything else to get the problem fixed.

Cheers, Ken