www vs. non-www not redirecting
www and non-www are handled automatically, there's nothing to configure. If something still seems off, it's almost always one specific thing: DNS.
How it actually works
Visiting either version of your domain, www.example.com or example.com, resolves to the same set of rules. On top of that, www always canonicalizes to the bare domain first: a visit to www.example.com/old-page gets a 301 to example.com/old-page, which then triggers your actual redirect rule to its destination. It's a two-step hop by design, not a bug, it keeps one canonical version of your domain in front of search engines and browsers.
If www doesn't redirect at all
This is the real, common cause: the www CNAME record simply isn't set (or is set incorrectly) at your DNS provider. If that's the case, traffic to www.example.com never reaches Redirect Supply in the first place, it fails to connect before any rule can run. This is a DNS issue, not a rules issue, see DNS Setup Guide to check it's in place.
Seeing two redirects in a row
If you check your redirect with a tool like curl -I, or notice it in your browser's network tab, you may see two hops: www to non-www, then non-www to your actual destination. That's expected. After the first visit, browsers cache that initial www-to-non-www redirect, so on repeat visits it's not noticeable, only the destination redirect happens.
Leftover records from a previous host
If www was previously pointed elsewhere (a CNAME or A record from an old host), it needs to be replaced, not left in place. See Record Conflicts for what to check.
Testing tip
Test in a private/incognito window or with curl -I rather than your regular browser, browsers cache redirects aggressively, which can make a fixed rule look like it's still broken.
See Redirects Not Working for more on that.
Still not working
Email support@redirect.supply with your domain, we'll check the DNS and rule chain directly.