This is a difficult one.. First you need to have a great quality coffee.. But seriously there are many variables here.
1. Firstly make sure whatever you are brewing in is clean
2. Use approximately 7g of Coffee per single shot
3. You need the water to pass through the coffee for 25-30 seconds, so you will need to press the coffee to slow the water flowing through is - Baristas call this Tamping - you will need to play around with this
4. Espresso should be served hot - Please use a Water temperature of 90 degrees
5. Serve immediately.
This is not an exact science, experiment with the coffee by adjusting various elements of the recipe.
Tip 1. If your espresso tastes sharply acidic, try to make the grind size finer or increase the time it takes to brew by compacting the coffee more (tamping)
Tip 2. If the espresso tastes bitter, try to coarsen the grind size or shorten the time it takes to brew by compacting the coffee less (tamping)