Your single is for one shot and a double is for two. Personally I don't like to make single shots. I like to make doubles for everything, but I also like espresso and the flavor it adds to Lattes. For both your single and double shots fill the basket full and then level it with a straight edge. Most of the shops we set up have a Mazzer Super Jolly and they use the lid to the espresso grinds to level the espresso in the portafilter. This usually will concave the espresso just a little, and helps prevent over filling.
You should tamp on a level surface, with around 30 pounds of pressure. This will also depend on you Oscar if it can handle it. From my experience with small espresso machines that use a vibrating pump some times you cannot follow the same method you would use on a commercial machine.
Your grind is going to depend on Oscar. If your grind is too fine then the espresso might drip out, not come out at all, or can be very slow. When your making a shot the espresso should fall straight down from the spout. If it curves in, your espresso is to fine. If it looks like is gushing out and somewhat coming out at an angle then your espresso is to coarse.
Until you get used to your machine you might try using shot glasses with the line painted on. That line should be about 1 ounce. Your goal is to fill to that line. It should take around 15 to 25 seconds. This time will vary with the type of espresso and who ever is telling you this! Some people say it should take up to a maximum of 30 seconds. Other say it should take exactly 23.5 seconds. You are going to have to play around with this until you find the perfect setting.
The end result that you are looking for is a nice colored shot. Not extremely dark and burnt looking or really white. For more help you should visit Nuova Simonelli’s website and read about making espresso here
http://www.nuovasimonelli.com/espressoc ... ink&Id=795
Remember though an Italian most likely translated this into English. At the very bottom it tells you about what happens when milk gets into the boiler and quotes you $500,000 to fix the problem. Also most places I have been go a little higher on steaming milk. NS has you stop at 140 so the milk gets up to 150 degrees, I go up to 160 for my use. 150 just seems to cold.
There are a lot of things that I haven't told you, fine tuning, good characteristics of espresso, cleanliness and maintenance of your machine, water quality, espresso quality, and the experience of a good barista. Play around with your machine and repost your problems here. Trust me when I say I won't be the only one who is willing help you here.
Remember it takes time, patience, a lot of a espresso, milk, and money to become a good barista!