Step one - Look at the wine

Tip it to an angle of about 450 and look at it against a white background.
Is the wine cloudy? If so that could indicate a fault.
Is the colour pale or deep? This could give a clue to the intensity of the flavour.
Compare the middle of the glass with the rim. You should see that the colour of the wine fades as it nears the rim. Young whites will go transparent, young reds pink. As the wines age they will take on a  golden or tawny brown note.

Step Two - Swirl and Sniff
Swirl the wine around in the glass (this is another reason for not overfilling the glass) and release the aromas from the surface of the wine. Stick your nose in and sniff gently.
Does the wine smell like wine? If it does, good! Wine faults tend to replace the nice wine aromas with something nasty.
What does the wine smell of? This is a deeply subjective process. We all smell things differently but wine aromas can be spilt into four main categories:

Fruits - Citrus, berry, tropical or stone fruits
Vegetal - Asparagus, grass, bell pepper or brassica
Floral - Rose, honeysuckle, violets
Spicy - Pepper, ginger, cedarwood, vanilla

Step Three - Slurp!
Now for the fun part that you have been looking forward to. Tasting the wine. Strangely, this is not the most important step, it merely confirms what your eyes and nose have told you. Because your tongue can only detect water soluble flavour elements, sweetness, bitterness, acidity and salt, (a new notion is that you can also taste savoury elements such as soy sauce) you have to slurp to get the best from your wine. This gets the wine fumes into a nasal cavity called the olfactory bulb which can recognise between five and ten THOUSAND flavour elements.

To do this, take a small amount of wine into your mouth, tilt your head forward (this should stop you spluttering) and suck some air through pursed lips and through the wine. Practise this, as a good slurp will earn you lots of respect in restaurants!
Does the wine taste the same as it smelt? Look for the flavours that you found using your nose. Is there just one flavour or are there a number adding ‘complexity’ to the wine? How strong are they? Are they in ‘balance’ or do some overpower the rest? How long do the flavours remain in your mouth? The longer the time the better the quality. Look also for the texture of the wine. Some are ‘watery’ yet others can have an oily feel to them.
But most importantly ask yourself;
Do I like it?

Step Four - To spit or not to spit?
There are two reasons to spit wine out, firstly if it is actually horrible or secondly, if you are tasting a large number of wines and want to keep some semblance of judgement (and remain standing). In all other cases enjoy the wine as it was meant to be enjoyed and feel the aftertaste and a glow of well-being.