Professional writing is critical for job search and performance, but many – especially those without work experience – struggle to write well. We introduce an instructional approach called ‘scaffolded annotation’ as a way to guide students in creating initial drafts of professional writing, like client emails and cover letters. We studied the implementation of scaffolded annotation in a digital platform called Lettersmith. First, we performed a quasi-experimental study and found that students applying scaffolded annotation in Lettersmith were more likely to include key components of professional writing. We also interviewed instructors and students who used Lettersmith and found that scaffolded annotation helped students in guiding structure, content, and tone. Instructors found the approach useful for articulating writing task expectations, pinpointing student gaps in understanding, and scaling instructional support for early-stage drafting. We provide implications for writing instruction and HCI researchers developing writing support tools.