Port is traditionally served after pudding with the coffee or cheese course. If you are at an informal dinner party or a restaurant no ritual will be involved, but take more care at a formal dinner when you should not take a sip before the Loyal Toast.
A port decanter will be placed on the table so that you can help yourself and then pass it on. Always pass the port to the left. If the port passes you by without your glass being filled don't ask for the port and make it change direction. Just send your empty glass after the port decanter and ask for it to be filled, or better still wait for it to come round again.