1. First, you have to be a descendant of the lady pictured above, Electress Sophia of Hanover. If you aren't, then you can just scratch your ass off the list right away.
2. You must be a protestant at the time of your accession. You must enter into communion with the Church of England after your accession.
3. Just in case you didn't get number 2, anyone who is a Roman Catholic, becomes a Roman Catholic, or marries a Roman Catholic (or maybe even just touches a Roman Catholic) is permanently excluded from the succession.
4. Congress shall make no law respecting an establishment of religion. Wait. That doesn't apply to British Monarchs. You must be a protestant. I can't repeat that too often, but I won't again.
5. A person who is born to parents who are not married to each other at the time of birth (a bastard) may not be included in the line of succession. ::Coff coff:: Subsequent marriage of the parents does not alter this. A child born to a married woman is assumed to be the child of her husband. ::Coff coff::
6. A monarch is always succeeded immediately by his or her legitimate descendants. Birth order matters. Gender matters: boys first, girls second.
7. Once you start, you can't go back: until you run out of children and grandchildren, your siblings are just out of luck.
"British throne" today means the throne of the United Kingdom of Great Britain and Northern Ireland.