以下内容来自Google:

Windows installation steps

0. Download the plug-in (by example from GitHub),

1. Copy the plug-in file WebPShop.8bi located in bin\WebPShop_0_3_0_Win_x64 to the Photoshop installation folder, usually C:\Program Files\Adobe\Adobe Photoshop\Plug-ins for x64.

2. Restart Photoshop. "Open" and "Save" menu commands should now show WebP files.

Mac installation steps

0. Download the plug-in (by example from GitHub),

1. Copy the plug-in WebPShop.plugin located in bin/WebPShop_0_3_0_Mac_x64 to the Photoshop installation folder, usually Applications/Adobe Photoshop/Plug-ins.

2. Restart Photoshop. "Open" and "Save" menu commands should now show WebP files.

On macOS 10.15+, the prompt "WebPShop.plugin cannot be opened because the developer cannot be verified" can be bypassed by running the following in Terminal (Finder > Applications > Utilities):

sudo xattr -r -d com.apple.quarantine "/Applications/Adobe Photoshop 2020/Plug-ins/WebPShop.plugin"

感谢:
https://developers.google.com/speed/webp/docs/webpshop