There’s a few ways to do this, I’ll explain the browser method since it doesn’t require additional application installation. Navigate to your site settings in your site collection root, then open “Master pages and page layouts.” From here, use the Files tab in the ribbon and click the “upload document” button. Upload your display template (it should be a .html file). If your template was created correctly, most or all of the fields should auto-populate with the right settings. Adding a preview is optional, I’ve never seen this actually show up anywhere.
Next you need to publish the file (this is important). The quickest way I’ve found to do this is to just select it in the library, check it out, check it back in and select the option to publish a major version.
That’s it! Your display template should now be visible from your Content Search web part settings.