hardest part for me is the very beginning, just getting some paint on the canvas. I use my fingers as little measuring sticks, trying to keep proportions as I look from photo to canvas. --as in, the widest part of her face in the photo is the length of my index finger from tip to main knuckle and on the canvas it's 5 of those units. It's definitely not scientific, but sometimes this initial proportioning helps me get the bare bones started.
I try to focus on just the light parts and then switch to focusing just on the dark parts. Rinse and repeat.
The fun part begins when the "structure" is
there and I get to start slopping color on, mixing, and making a mess. I have had to learn that sometimes, no matter how much I want to live with it, when something isn't quite looking right to me, it's worth it to go back and redo it...if I can even pinpoint what's looking goofy. I went back and redid her right eye after it was all detailed and nice (ugggghh) because it turned out being just a smidge too close to her left eye. Helped a lot...and the painting quit nagging at me, saying "I'm just not quite right."
Once I can finally pry myself away from touching up here and adding something there...it's true, an artist's work is never done...and I get to the point where I can live with it if I never touched it again, I put a little signature on it. Mine changes just about on every painting lately :)