How do I charge for posts with a paid membership?

Open Memberships (under Income in the sidebar). First connect a Stripe account (setup takes a few minutes; if you already use Stripe you can sign in with it). Then, for each website, turn on Offer Paid Memberships and set a monthly price, an annual price, or both, from $1 to $10,000.

Now set any article's Who Can Read It to Paid Members. Visitors and free members see the opening of the post plus a subscribe card with your prices; a reader signs in with their email, picks a plan, and pays through Stripe checkout. The money goes to your own Stripe account, and you handle payouts and refunds from your own Stripe dashboard. A paid member reads every paid post on the site, and Send to Members emails a paid post only to paying subscribers. Subscribers manage or cancel their plan through Stripe's billing portal, from the same subscribe card once they are paid.