Prior to WordPress 5.8

• Install and activate the Gutenberg plugin.
• Install and activate a Full Site Editing theme – see Themes
• View the site.
• Click on Edit site to start using the Site Editor

With WordPress 5.8

Either install Gutenberg and continue as above or

install an FSE theme and make use of the functionality that is available.

With WordPress 5.9

To be completed.

Recommendation: Work in a development / test environment

Since Full Site Editing is still experimental working in a Live site is not recommended.

You could try working in a staging site or in a local development environment.

References

localwp.com – Local WordPress development made simple.