So here is the basic method...
In a lined/squared/dotted notebook (most prefer A5 size):
The first two double page spreads will be your index. This is an important and useful feature of the Bullet Journal. Every time you fill some new pages in your journal, write them in the index.
Number all the remaining pages!
Pages 1-2 or 1-4 are your 6 month or year overview (known as a Future Log). Divide the pages into 6 or 12 (depending on how many months' journaling you think you'll fit into your notebook) and label them with the months. Write in any events/appointments which have been planned in advance for those months.
Next double spread is your monthly calendar and task list for the first month of your BuJo. Write the dates down the margin of the left page. Use this as your calendar for any events this month. Right hand page is your To Do list for the month.
After that, the following pages are your daily journal for the month. Some busy people like to use a whole page per day and include their work meetings, after school clubs, what they have for dinner, what the weather's like, etc etc.
Others (like me) take a week-to-view approach and divide their double page spread into 7. (Pics to follow).
Apart from your calendars and daily/weekly journal, you can fill up your BuJo with all sorts of things - lists of diy projects, craft ideas, supermarket shopping master list, holiday planning list, list of books you read or want to read. The possibilities are endless.
The key thing is, you can add a new 'collection' or page of stuff any time you like. Just turn to your next blank spread and dive in, even if you are right in the middle of a run of daily journal pages. Just index each page you fill, and you'll know where everything is!