First off, not all wine needs to be decanted. Most wine purchased off the shelves these days have been filtered and fined to the point of crystal clear clarity. However, many older wines (typically red wines, 10+ years old) will have unsightly sediment (solid matter) that has settled at the bottom of the bottle. Though the sediment is harmless, it definitely gives the wine an unsightly appearance and taste.
If you are decanting the wine for appearance sake or to let it aerate, just pour it straight into the decanter and enjoy. Otherwise, begin by setting your port upright on the counter 24 hours before you plan on serving it.
When you are ready to serve, very carefully, remove the cork from the bottle. Try to keep the bottle flat on the conter rather than holding it with your hands or arms while removing the cork. This will stir up the sediment and make it more difficult to pour. If the bottle does get shaken up a bit during the process, don't fret. Just set it on the counter and let it rest for approximately 20-30 minutes.
With your decanter nearby and the lights dimmed, light your candle and place it besides your decanter. Hold the bottle with both hands directly over the candle so it illuminates the wine as it passes through the neck of the bottle. Very slowly, but in one smooth motion (as not to stir up the contents of the bottle), pour the contents into the decanter until the wine in the neck of the bottle begins to get cloudy or shows signs of sediment.
Get ready to enjoy your port! For best results, serve immediately. Make sure it doesn't get too warm, and try to drink it as soon as possible. The shelf life of uncorked ports can range from 24 hours to a few weeks, depending on the age and quality.