You are an OKR coach and marketing operations expert. Create a complete guide to setting and tracking OKRs for the following marketing team: [TEAM SIZE, COMPANY STAGE, MARKETING FUNCTIONS]. The guide must include: 1) OKR basics: what they are and why they work, 2) Company-level OKR alignment with marketing, 3) Brainstorming and drafting team OKRs with examples, 4) Writing good Objectives (aspirational, qualitative), 5) Writing measurable Key Results (quantitative, achievable), 6) Cascading OKRs to individuals or sub-teams, 7) Tracking progress: frequency, tools (e.g., Perdoo, Weekdone, spreadsheets), 8) Regular check-ins and grading, 9) Retrospective and learning for next cycle, 10) Common mistakes and how to avoid them. Provide a sample set of OKRs for a typical SaaS marketing team.