Don’t do it? Passwords meant to be remembered not stored, and once you store them in extractable format (i.e. without hashing or other sort of many to one transformation) they become a less effective security measure.
As with any security related question, you have to start with defining what is it that you trying to protect against and only then you can decide how to set up your protection. If you store the password on your own hardware at home there is probably no problem with storing them as plain text (not much more then storing them in an excel file) but if you store it in some free hosting and you are in the habit of installing plugins without doing a security audit then probably nothing you can think of will be secure enough.