To install a Cloudflare App, visit the Apps tab of your Cloudflare dashboard. Select the App you wish to install and click "Preview" to view the App's Live Preview on your site.
Inside the preview, you will have the opportunity to choose the options which suit you and potentially create an account with the service which powers the app if applicable. The preview you see on the right side will match how the app will perform on your site if you choose to install it.
Choose which pages on your site the app will appear by clicking the "All pages" button at the top of the page.
When you're ready, click "Install" at the bottom of the page to add the app to your site.