package files in the Sims 4 Studio Mods folder, you can use the Search function or the other drop-down menus to find the exact skin you're looking for. Use the Content drop-down menu and select Custom. Click the bubble next to Create 3D Mesh, then click CAS above that. Here's an unnecessarily large screenshot to help you:
package file for the skin to your Sims 4 Studio 'Mods' subfolder.
