First, compose a phrase or sentence that you can easily remember and others are unable to guess it.
「瀬戸内海、五月晴れ」
Pick the first character of each syllable in the phrase or sentence.
Setonaikai, Satsukibare
Stniki,Stkbr
If the resultant password does not include a digit or a special symbol,
then insert it or replace a character of the password with your own rules.
Stniki,5tkbr