Two steps: create and deliver
Create: the "Create WebP" action generates WebP versions of the original and all thumbnail sizes — alongside the originals, which are never touched. Enable "Create WebP in parallel" in the settings to create WebP automatically with every compression.
Deliver: enable "Serve WebP to visitors" under Settings → Compression. The plugin rewrites the page's images to picture elements with a WebP source — the browser chooses, and older browsers automatically get the original format.
Why picture tags?
The alternative (.htaccess rules) only works on Apache and conflicts with cache plugins and CDNs. Picture tags work everywhere, are cache-safe and require zero configuration.
Verify it works
Open a page logged out, right-click → Inspect. Images should sit in <picture> elements with a <source type="image/webp">. The Network tab shows webp files loading.
Limitation
The rewrite covers the page's rendered HTML. Images loaded via JavaScript afterwards (some sliders/galleries) are not covered.