Suppose you want most of your site to be free, but to sell access to premium content in a special area. A simple way to do this is to make a premium category which only paid usergroups can view.

Start by creating your premium usergroup at Admin Panel -> Members -> Manage Usergroups. You can just use the existing "Sponsor" usergroup for it, unless you want to have multiple levels of paid membership -- but you'll probably want to rename "Sponsor" to some more appropriate description like "Paid Member" or "Premium Member", so go ahead and do that.

Next, go to Admin Panel -> Revenue -> Sponsorship Levels and click "Add a Usergroup Sponsorship Level". Select to promote to the premium/sponsors usergroup you just set up, and to revert to the usergroup Member. Set the price as desired (say $2 for 30 days for example). For the "level description" field, type a short pitch that will tell people the benefits they get from buying this level of membership.

In WSN 9.2.68+/10.0.19+ and later, WSN will automatically redirect people to the member sponsorship page when they click on a premium category while logged into a non-premium account. If they're not logged in at all, they'll get the login page.