Before you start

Step 1: Stop and work out when it broke
Restoring to a point after the problem started just reproduces the problem. Check the error log and your own timeline — an update, a publish, a plugin install — and pick a restore point from before it.
Step 2: Decide what you are restoring
Files and database are separate decisions. A broken plugin usually needs files only; a corrupted post or a bad import needs the database. Restoring both when you needed one loses everything written in between.
Step 3: Save anything written since
Orders, comments, form submissions and posts created after the restore point will be gone. Export them first if they matter. This is the step people skip and regret.
Step 4: Restore, then clear every cache
Server cache, plugin cache, and any CDN in front of the site. A restore that appears not to have worked has usually worked fine and you are looking at a cached copy of the broken version.
Step 5: Find the cause before re-applying anything
You have rolled back to a working state, which means the thing that broke it will break it again if you simply redo it. Reapply updates one at a time on staging.

The mistake people make

Restoring the database to fix a plugin problem. It fixes the plugin and deletes every order taken since the restore point.