In my opinion, the best way of getting started is to create a clone/staging of your existing site to avoid any downtime.
You could ask your hosting if they have any such ability to create it for you. If the hosting support cannot help you with this, there are lots of tutorials out there like this one https://smartwp.com/clone-wordpress-site/ .
You can now start off by installing and activating Rey. The 2nd optional step is wether to import a pre-defined demo.
Please know that when importing, the following data will be added into the site:
- Products & Categories
- Blog posts
- The main menu can be set to a different one (that has just been imported)
Most certainly you most likely won’t need that. So you will eventually need to make a cleanup and remove this new imported data.
Normally no extra steps should be made. Just by switching the theme to Rey, should comply with all of its stylings.
Where you will likely need to look into, is: