Most of the time, in modern American samplerooms, the patternmaker pulls an existing pattern (or block) and makes a modified copy of it to match the new style, either on paper or on computer.
If the style is completely new, the patternmaker will usually drape a rough draft in muslin fabric on a dress form, then show it to the designer to discuss any changes before transferring the markings to paper to create the pattern for cutting.
Patterns may also be drafted from measurements, this method can also produce well fitting garments as long as the patternmaker has a good handle on shapes and balance.
Apprenticeships are almost unheard of in modern America, but would serve well to improve the transition from student to professional status.
Because this occupation is relatively unknown outside of the apparel industry, there is a serious lack of patternmakers who can accurately interpret designs in Los Angeles, and possibly other fashion capitals.