Whilst Mr Snowden's advice is good, the need is usually for different passwords for multiple sites and remembering that is hard. But other ways can also significantly alter a 'known' word, such as adding punctuation - so Flyertalk (9 chars) becomes F.l.y.e.r.t.a.l.k. (18 chars) or Flyer;;;;;talk (14 chars). Unguessable and not going to work as a brute force attack anytime this century. This Steve Gibson explanation of needles in haystacks is fun:
https://www.grc.com/haystack.htm

Whilst handy for extending and complicating an easy password it only gives you one example and if a site gets compromised and you have used that password elsewhere you have a problem. Many sites store your email address and password and it is if that single site is compromised that causes problems as you probably use the same email address for all your logins - hence why different passwords for each site are important.

But the password manager advantage is that it will give you a randomised 20 character password that is different for each site. If one site ever got compromised the email address and that password does not help the hacker get in anywhere else.

Even if you keep those passwords encrypted on your phone and have to manually input it to keep an employer happy it is way better than the post-it beside the monitor!