Opt-Out from newsletter

but then how do I create the flow step 'if the user clicks this link in the newsletter, then add them to the Exclusion Smartlist?

You don't.

But that's for multiple reasons, the primary one being that you must not treat a Clicked Email activity as being done by a human. Clicked Email may be the result of human activity or mail scanner activity. And as a result, though this wasn't true say 5 years ago, you cannot offer "one-click" anything anymore. Unless you really don't care about people being removed from mailings when they actually didn't click anything... and I strongly doubt you want that!

So fundamentally, you must require the lead to perform a humans-only interaction with your page. And in this case, that means clicking a button to submit a form. The form need not have any other fields on it, just a big ol' button. But only if somebody clicks that button do you know that, well, somebody clicked the button, instead of somebotty.

Do I create one campaign to send the newsletter to the 'Newsletter Recipient' smartlist minus the Newsletter Exclusion smartlist, but then how do I create the flow step 'if the user clicks this link in the newsletter, then add them to the Exclusion Smartlist?

There's no concept of Add to Smart List in Marketo. Smart List qualification is decided by SL filters, which in turn may refer to field values, activity log entries, segment membership, list membership, program membership, or any combination of these factors.

So instead of "add to Exclusion Smart List" you must mean "ensure the person qualifies for an Unsubscribed Smart List".

And there are, as you can see above, a lot of ways to manage subscription preferences in Marketo. There is no one best way for all people. But I recommend you explore program membership for this task (rather than field values or static list membership). You'll learn a lot more from the docs and past posts than I could possibly recite here.