To source on AngelList you'll just need to be sure you have a login setup, there is no charge to join AngelList.
Once signed in you can use the search bar in the upper right to look for candidate profiles matching search terms, such as company name, skill set or job title such as:
From the results page you can then click on People to filter to only profiles matching your search parameters.
From there click on any profile to open up their profile for sourcing. Once you've loaded a profile page for any particular individual, click the Resource icon in the Chrome Toolbar to open up the sourcing panel!
Click Add to Campaign and you're all set!