Sketch for Confluence is available directly from the Atlassian Marketplace and can be installed on your Confluence by going to "Find new apps". Only Administrators can install apps on Confluence.
The installation process will be slightly different if you're using Confluence Cloud or Server. Here are step-by-step guides for both:
Direct from the Atlassian Marketplace:
Or install directly on your Confluence Cloud instance:
On your Confluence Cloud navigate to the homepage by clicking on the Confluence Logo.
Then click the cog icon to navigate to the Confluence settings admin page.
Scroll down to find the section Atlassian Marketplace and click on Find new apps. On the marketplace search bar look for Sketch for Confluence
This is the direct link for the marketplace: (Change <YOUR_CONFLUENCE_URL> to your Confluence URL)
https://<YOUR_CONFLUENCE_URL>.atlassian.net/wiki/plugins/servlet/upm/marketplace/search?q=Sketch%20for%20Confluence