Select your pattern size. I tend to re-measure before every project (in the underwear I'll be wearing with it). I'll compare my measurements to the pattern measurements for the finished garment, not the sizing bands and select the size I need to cut out: I do this as I generally like a bit less ease in the bust and waist and more in the hips... and I'm a bit of a funny shape!
I tend not to make any alterations to the initial pattern to allow me to see what's what. For an important or difficult to fit garment, I'll make a muslin (or trial garment) to see what alterations I need to make to the straight size.
Anyway. Once you've got your pattern size selected, cut out your pieces.
No, really. That's not terribly helpful. Get off.
As I was saying, once you've got your pattern cut out, you might like to make a test version. I like to make mine out of a similar weight fabric and will try to make them as nicely as possible so I can wear them.
I like to cut my fabric doubled, because I'm fundamentally lazy and it saves a bit of time. At the moment the dining table is buried under a load of junk, so I spread my fabric carefully out on the carpet. Be sure to ease out any creases or wrinkles before you lay out your pattern and...
...oh, for goodness' sake.
Guys. You are really not helping here.
No, I don't need supervising, I am actually allowed to use the scissors.