Most people feel more comfortable using some kind of filter: either a purpose made funnel, or other such material eg. coffee filter paper. However if the bottle of Port has been properly prepared and addressed, such items are not always necessary.
If using a filter, people tend to pour the entire contents of the bottle into the decanter - assuming that all the deposit will be removed. At times the sediment in a bottle can be quite fine and passes through some filters leaving the Port somewhat cloudy. For this reason I prefer the following method:
A day before you intend to decant, place your bottle of Vintage Port upright. This will allow any loose sediment to fall to the bottom of the bottle.
Obtain a clean, odourless container. A decanter is ideal. Alternatively you could decant into another container, rinse out your bottle and return the port to the bottle.
When pouring the Port into the container it is important where you hold the bottle. When the Port has been lying in our cellars in Vila Nova de Gaia, we will have marked the uppermost side of the bottle at the bottom with a white mark, called the splash mark. The reason for doing this is that over the years sediment will have formed in the bottle and settled on the lower side. Some of this sediment can stick to the glass so that if not careful when decanting, air bubbles can dislodge this material and it will ruin the effect of your having stood the bottle upright for a day.
To avoid the risk of this happening, when decanting, always place your hand on the same side of the bottle as the splash mark. In this way, any sediment stuck to the side of the bottle is below the wine and away from air bubbles entering the bottle as you pour.
At times, however, this splash mark has been wiped off, which is why at Taylor's we will always put the front label exactly above the splash mark. In this way all you need to remember is that when you pour, have the label in the palm of your hand.
Now you must hold your decanter so that when you pour the port you can see it briefly passing over the neck area of the decanter. This is because, as you pour with your steady hand, you are looking for signs of the deposit starting to come through. Stop as soon as the sediment starts to enter the decanter.
You will need a light background to see the sediment clearly. In the past, candles have been used for this.
Pour in one steady movement. Stopping part way through will stir up the sediment and you will need a filter from there on in.
I suggest you decant before the meal, you are likely to have a steadier hand and it will give all but the very oldest vintages the required time to breathe.
Finally, do not throw away the sediment. It is very nutritious and excellent in soups, sauces and gravy. I freeze mine in freezer ice cube bags.
It seems a long explanation, but once understood (and practised) you'll soon be a master.
[By Andrew Costello of Taylor Fladgate - click here to see their wine's on Snooth.]