Then click Download in the pop-up window.Then click View when you find the right OS.Before that, there was Mojave, High Sierra, Sierra, El Capitan, Yosemite, and Mavericks. Search for the OS you want to install on your bootable USB.Then click Applications in the left sidebar of the Finder window that opens. You can do this by clicking the Finder icon in your Dock. Note: You will need a flash drive or external hard drive that has at least 8GB of storage space. The command you enter will depend on the OS you want to install. Then use a Terminal command to install the OS on your USB drive. To make a bootable USB, you need to download the OS from the App Store. Here’s how to make a bootable USB on a Mac. It can also help if you want to install the OS on multiple devices. This can save you if your computer ever experiences problems connecting to the internet or booting in Recovery Mode. When you have a bootable USB with your OS (operating system) on it, you can install it on a computer without having to download it from the App Store.