The tutorial linked above is excellent, it helped me massively when I had a second bite at home espresso, and got far better results than the first time around.

One thing that no one has mentioned is sour - bitter confusion. It's quite common to confuse these tastes, and find yourself chasing between under and over extraction without understanding which direction to go in.

As far as process goes, I'd recommend eliminating as many variables as possible. Use nothing but a bean that is fresh, and something that you're familiar with from a good cafe - this is your reference for taste. As previously mentioned, medium dark is often a good point to aim for all it's quite easy to extract. Leave the flow control alone for now - set at 4ml/s or so and don't touch it during the shot. As previously mentioned, the stock double basket and a 16g dose is a good starting point, but the most important thing to do is to set and forget - the only thing that you want to change for now is the grind size.

Set the grind deliberately too coarse, so that the shot runs in 12 seconds or so for 32g out. Taste it, it should be very acidic. This is under extraction. Pull a series of shots with an increasingly finer grind, trying each of them, and noting the change in flavor. You should find that the acidity reduces as you get to the 25 second mark or so. At some point, you will find that the shots start to take a more acrid and bitter characteristic. This is over extraction.

Most of the difficulty in espresso is finding where the sweet spot between under and over extraction is. Some coffees are harder than others to dial in, but this is the basic procedure. Good luck!