You go to the Create Account page and create the account as described there. There is currently no way to link your primary and secondary account to make managing them easier, but that may be a future feature.

Alternatively, you can create a community, which you can manage from your primary account by being owner and moderator. However, a personal account and a community can not be used in the same way, and depending what you need a secondary account for it may not be the best option.