For payment it depends on the client. For many folks, they are using the Coil plugin. You deposit a small amount of money in Coil ( say $5 a month) and it will know when you are on a site that accepts web monetization and send drips of pennies to that site as you spend time there. https://xrpcommunity.blog/how-to-monetize-your-content-with-coil/
You can add the meta tag to your HTML content but Itch.io has that in an IFRAME. So Itch.io needs to add an attribute to that Iframe to allow web monetization: https://webmonetization.org/docs/api#iframes
<iframe src="/wm/example.htm" width="600" height="800" allow="monetization"></iframe>