This short guide is just intended to get you started. There are several excellent books in our products section that go into far more detail!
Always use good quality roasted coffee beans that are as fresh as possible, and a good burr grinder (rather than a blade grinder) that has a good range of adjustments.
Fill the larger (double shot) filter basket without tapping the handle or pressing the ground coffee down. Level off the loose ground coffee by sliding a straight finger (or something similar such as a knife blade) back and forth across the top edge of the filter basket filling any gaps in the ground coffee and brushing off any excess (this should give you approximately 14 to 17 grams of ground coffee in the filter basket). Next you need to "tamp" the coffee, ideally using a good solid hand-held tamper. The first press with the tamper should be very light and will leave some ground coffee up the inside walls of the filter basket. Firmly tap the side of the filter basket with the back of the tamper to topple this wall of ground coffee into the filter basket and to knock any remaining loose coffee off the top edges of the filter handle. Brush off any excess ground coffee that is not inside the filter basket with your finger. Then firmly tamp the ground coffee down into the filter basket (experts recommend 30lbs of force) making sure to keep the bottom of the tamper horizontal, twisting the tamper as you release the pressure so as to "polish" the surface of the tamped coffee. Finally fit the filter handle onto the group head and you are ready to draw your espresso shot.
You should measure the amount of coffee that is extracted by placing a shot glass with a line at 1 fluid ounce under one of the spouts (as you are using a double filter basket you can place an ordinary espresso cup under the other spout). You then need to time from the moment you operate the switch/lever for drawing your espresso until the moment the top of the crema reaches the 1 fluid ounce line on the shot glass. This should take 20 to 25 seconds. If the espresso comes out too fast (takes less than 20 seconds) you should adjust your grinder to make the ground coffee finer, and if it comes out too slowly (more than 25 seconds) you should adjust your grinder to make the ground coffee more coarse. Keep repeating this process until you obtain 1 fluid ounce in 20 to 25 seconds and then you will have calibrated your grinder and will be getting the best espresso you can from your machine. If you have a group head pressure gauge on the machine you should now find it shows close to 9 bar while you draw your espresso.
The procedure is identical if you are using the single filter basket except you will only be using around 7 to 8 grams of ground coffee.
You should repeat this calibration process regularly as it is affected by the type of coffee beans and their freshness and, to a lesser degree, the ambient temperature and humidity.