i want to clean install my macbook pro currently running Yosemite. It's el captain compatible. my main question is how do i download and save the el captain file from the app store? I see it in the app store and when i press "get" it prompts user password. I haven't done it yet because i don't actually want to start installing it on my computer right now i just want download it in order to make a bootable usb. Any suggestions and tricks? Thank you and i know this is probably pretty basic problem for most.