Comments may be in your way at first. Read them and discard them.

Add rows and columns where necessary. Be careful to copy formulas correctly!

Note: when editing, you'll need to unfreeze rows/columns. View -> Freeze

Fill in each cell where you achieve a milestone. Milestones refer to the beginning of the month where they're listed. Delete all rows above this one if not including your production schedule.

Fill in with cells of solid colors: Pre-Production, Production, Post-Production. Some games will launch directly into production (depending on submission stage). All games have post-production.