I first come up with one or two personality factors - serious, playful, bookish, a prankster, haughty ... Then I ask myself what I want her/him to value - intelligence, strength of character, fun, blood purity, friendship/popularity ... Then I ask myself if my character is maybe too much like myself. If so, back to the start, because this is a fictional character, not me.
Next is the fun part - what my charrie looks like! Mostly her/his personality comes through after this step. Hair, eyes, build, face, markings ... the whole bunch!
Family is usually next - what is my charrie's background? What is her/his blood status? What was her/his upbringing like? Parents? Siblings? Guardians? Second-cousin-thrice-removed?
Lastly is the naming. I try to name my charrie in accordance to what she/he is like - ie, the meaning of the name. For instance, a shady charrie with a lot to hide might be called Shadow.
Here's my current charrie Jezabel's personality steps. I've put the last step first, because I always knew she was going to be Jezabel. Just knew it.
Name: Jezabel Khalida Black - Jezabel is a Hebrew name and means "not exalted" (this refers to Jez's bad status in her family, who are prejudiced against Muggles and the like); Khalida is an Arabic name meaning "eternal" or "immortal" (Jez is kind of arrogant in that she sometimes believes she can do anything she likes and not get into trouble); Black is, well, black, and refers to her family (and Sirius black, my favourite character in HP!)
Factor(s): Intelligent and playful
Values: Intelligence
Looks: Jez has straight waist-length black hair, and violet eyes. I saw this look on a teenager in the mall a while back, and she looked gorgeous! All male eyes turned on her - literally. She is slim with an hourglass frame. She has a scar running up her from her left hand to elbow from breaking it in a broomstick accident.
Family: Jez's family are purebloods and completely horrible - blood purity, wealth and being in Slytherin are their only values, it seems. Jez hates them, expect for her 18-year-old brother Keegan and 9-year-old brother Than, because they, like her, have escaped the family influence. Jez has 6 siblings, which means there are 7 kids in the family - this was no accident on my part! I wanted Jez's parents to be so bent on magical purity they had 7 kids (7 is considered the most powerful magical number).
Hope this helped!