Go to "Create a List." Under the Home District field, select your County. To select multiple counties, click on the word Counties, highlighted in blue. This will then take you to a page where you can check as many counties as needed for your search.
Once you select a county, the page will refresh, and you will have the option to choose a precinct under the new dropdown. Again, to select multiple precincts for one search, click on "precinct," highlighted in blue.
From here you can narrow, remove, or add people as needed.