Check your new barrel. The tap should start easily and be screwed in tight to the face of the barrel.
After the initial inspection for chips and cracks, fill your barrel with water, and let it sit for a few days to soak. If your barrel does leak initially, don't panic; the oak expands as it absorbs water, which should plug all leaks.
Pour out the soaking water and rinse your barrel several times with clean water. You may choose to initially cure your barrel with brandy, and let it sit for three days. Some people prefer the taste of the resulting product but it is not necessary.
You can now fill your barrel with ports of your choice. As is standard preactice with the blending of most fortified wines, the aim is to get a broad range of ages in the one mix. Tawny ports, although cheaper, are already mixess of old and new ports, blended by the winemaker. The ideal situation would be to find some nice older vintage port and blend it with some moderately aged vintage port followed by some young port.
The young port will often be the toughest to find, but a snoop around your nearest warm climate wine region (Swan Valley, Rutherglen) could find you some good value young port.
The best blend always comes from the best ingredients, remember you can never salvage something undrinkable. Blending with brandy, whisky and wine should be avoided with good ingredients, but feel free to experiment with cheaper batches.
Keep your barrel topped up at all times. Oxygen is the greatest killer of any wines potential, even ports. While it happens much slower in fortified wines, there is a marked difference between fresh and oxidised ports. Oxygen turns ethanol into acetaldehyde, a key culprit in hangovers. So letting oxygen in gives you less alcohol, less flavour and more hangover!
Store your port for as long as possible, while keeping an eye on the ullage (air space). Ideal storage conditions are 17-18°C and 70% humidity. The dryer the environment, the faster the evaporation. As ethanol has a lower boiling point to water, the ethanol will evaporate fastest. If the alcohol content gets too low, the risk of spoilage increases.
If you ever have a sneaky taste, be sure to top your barrel up. Best to buy a resealable 200ml wine bottle and take a full bottle sample to try every now and then. That way you can have your barrel topped up and show all your guests your master blending skills.