The simple answer to this is on the low side.
Here are the steps you need to follow to successfully charge the AC on the low side
Step 1.) Look for any leaks
This can be easily accomplished by spraying the coolant pipes with soap water. If it starts bubbling anywhere, it means that there is a leak and it needs to be fixed before recharging the AC.
Step 2.) Opening the valve on the tapping fitting:
This is necessary as otherwise, the pin inside the tap can puncture the can containing the refrigerant damaging it and leaking all the coolant.
Step 3.) Connect the tapping valve to the can:
Make sure that it is threaded securely. Once it is securely screwed in, close the tapping valve that you opened in step 2 completely shut.
Step 4.) Remove the air in the hose:
Loosen the valve that connects the hose to the valve and let the refrigerant force out the air. Make sure not to get the coolant in contact with your skin to avoid a cold burn.
Step 5.) Switch on the car and put the AC on coolest setting and the highest fan speed
Step 6.) Open the valve to the low side and let the can dispense its contents.
Some precautions you have to undertake is not to allow the coolant to come in contact with bare skin. Wear eye protection at all times. Do not overcharge. A good way to determine this is by keeping an eye on the low-pressure gauge which should be at about 30 psi.
The AC can be charged on the high side if the car engine is not running but we do not recommend it.