ok generally on this forum we want more specific questions with enough to duplicate the problem. this is too broad.
Having said that, you would need a table with at least the following fields
CompletionDate
Timeline_ID //parent timeline id since I assume these will be groups of 12
Description
Completed //boolean 1|0 - maybe you don't need this I don't know
you'd also need some type of table called timelines
ID
User_ID //the foreign key of the user on that timeline
DateStarted //maybe you need that
DateCompleted //this might be needed to be filled in
You need to do some studying about relational database structure - if you're not familiar with this and you launch right into your project, you'll pay for it in the end. Spend a few hours learning relational db concepts esp. primary to foreign key relationships
Samuel