Clean install of El Capitan on 2011 iMac

I've just bought a new iMac and would like to give my 2011 iMac to a friend. I would like to give it as if it had been bought at an Apple Store--El Capitan as the operating system with the apps which come with the purchase. The older iMac is running Yosemite.

I've done this once before, three years ago, but the Missing Manual on El Capitan suggests that a clean install is much more difficult.

I'm requesting any help anyone can offer to make this process as simple, straightforward, and speedy as possible.

My thanks in advance,

John Kendrick