When done correctly the espresso will have a dark thick crema, carrying all the subtle flavours of the espresso into the cup.
Tamped incorrectly however leaves a bitter overpowering taste which is unfortunately usually covered up with sugar, syrup and milk.

The first point to note is to make sure the coffee beans are fresh roast and freshly ground.

Grinding should be done only when the coffee is ready to be made.
The oils in the bean are released once the shell is broken and the grounds will quickly go stale.

An on demand grinder is perfect here as coffee can be ground as and when it is needed at the touch of a button, providing fresh coffee grounds with no waste.

The coffee is ground into the group head in a 7 gram measurement for a single shot of espresso and 14 grams for a double.

Before tamping the coffee should be leveled, this is usually done by running a curved finger over the head to level the coffee with the top of the rim.

A Coffee Tamper

The first tamp should be quite light, around 5 pounds of pressure, to form a puck shape from the coffee grounds.

A good way of gauging the amount of pressure needed is to practice on a bathroom scale.

The final tamp should be around 30 pounds of pressure to leave a compact coffee puck to give the desire resistance to the water.

While still applying the pressure the tamp should be twisted through 720° to leave a smooth surface on the coffee, essential for even distribution of the water through the coffee.
The coffee should also be level as if at an angle the water will push through the low side giving a bitter taste to the coffee.

If done correctly the extraction should take 20-25 seconds and leave a nice crema and produce all the subtle espresso flavours.