First of all, visit the Retcon Wiki and sign in.

From the Timeline page:

1) Click Edit.
2) Scroll down to where you want to insert your entry.
3) Click in one of the cells on the row below where you want your entry to sit.
4) Right click, however over Row, then choose Insert Row Above.
5) Add your text: Column 1 is the date, column 2 is the story, column 3 is any new characters introduced, and column 4 is the description of what happens.
5a) Note that as it's a timeline of the entire universe, you can add to existing dates. Just don't overwrite anything already there!
6) To add a link, highlight the link text, click the link button, then fill in the form as appropriate.
7) Once you've finished adding your entry, type a summary of what you've done in the Summary box (e.g. "Added events of Phoenix 2") and click Preview.
8) Double check everything looks OK, then click Publish.