Note, these docs are for ExpressionEngine 3! Legacy (EE2) documentation can be found here.
Unzip the downloaded zip and place the 'seo_lite' folder in /system/user/addons/.
In the ExpressionEngine Control Panel navigate to the Add-On Manager and click "Install" next to "SEO Lite" in the list of add ons.
After installing SEO Lite all your entries will have an additional Publish Tab called "SEO Lite" (note that you can rename this in your config).