The question wasn't that hard, but they didn't seem to like the answer at all, which was along the lines of 1) see what you can re-use or adapt from existing WBS from previous similar projects; 2) meet with project team, work groups, stakeholder groups (depending on project and scope) to brainstorm tasks, organize according to categories, verify and validate accurate and complete; 3) document; 4) review and approval from project team, sponsor.