1. Start on the right
Draw and edit your first pattern
The rule starts out completely empty, with no cells being born or surviving. Begin by creating or editing a sequence tab on the right. Editing G0 defines the starting pattern there, while editing later generations changes the rule so the previous generation evolves into the result you paint.
That is the core rulegolfing loop: define a target, make a rule edit, and see whether the next generation behaves more like what you want.
Each tab in the tab bar is a specific pattern that you can control. Use Ctrl+N or press the + button to create a new tab. The greyed out cells are there to prevent you from accidentally breaking previous patterns.