The key page in your bullet journal is essential and usually one of the first pages! In my 6th bullet journal this is the key page I created.
The first box has a list with all the events that I want to track: appointments, ideas, important things, tasks… The second box reveals the status of each event: completed, transferred or cancelled. The last box is a colour key. I use different colours for personal stuff, for my work, for my hobbies etc.
Combining all these together, one task can be pink and completed and so on. Sometimes I forgot to colour or state the status of each task. But again, I don’t worry and I keep on going, hoping next time I won’t forget!