In order to use the RSS capsule you need to go to the website from which you want to take the RSS Feed.
You copy the RSS Feed address to your clipboard then paste the URL in the appropriate box in the RSS Feed Capsule.
Then select the number of posts you want to show in the capsule, at the present time 8 is the maximum.
Select how much of the post you want to show. I personally only use the title and first sentence. You can use the whole post but if you use the maximum number of posts and post full length post the RSS Feed becomes too large and few people want to read it. Making links short lets people see what is on your feed and they can see what interests them.
As you enter the size of posts, don't forget to test the feed by clicking on preview feed. This allows you to see how the feed looks and allows you to change it before you add the capsule to your hub.
After the capsule is set-up place it in your hub and the feed will then automatically update each time the website you took the feed from is updated. Sometimes there can be a delay of a few hours, this is normal and depends on how often the website is looked at by the feed server.