Just follow the steps indicated on the screen. Begin the installation process as usual.In the top menu, click on 'Utilities' → 'Disk Utility' → Select the main disk of your Mac → Press delete.

Your Mac will restart directly from the installer OS X The Captain 10.11 Made the copy, go to 'System Preferences' → 'Startup disk' → Select the bootable USB you have created → Press 'Restart'.Make a backup with time Machine (or how you usually do it, although due to its extreme ease and comfort I always recommend doing it by Time Machine).Now remove the OS X installer from your Mac (it's in the 'Applications' folder).It's very simple and with just three or four clicks you will have your El Capitan bootable USB ready. Opens DiskMaker and follow the process.Plug the USB of at least 8GB into your Mac.When the installer of OS X El Capitan 10.11 has finished downloading, close it.While it is downloading, check your Mac: put everything in its place, check the downloads folder, delete what you don't need and pass a 'clean' of the type Clean My Mac.At the same time, also download the full OS X installer from the Mac App Store, remember that there are a few say so it will take a while depending on your internet connection but in the meantime, you are preparing.Update your Mac to OS X El Capitan 10.11 And meanwhile, look at home for a pendrive of at least 8GB that you don't need, you will need it.The quickest and easiest option is to update however, if it has been a long time since you performed a clean installation, from scratch or, worse still, if you have never done it, the time has come you will gain space and, above all, you will gain in fluidity and efficiency, and this, together with the performance improvements of OS X El Capitan that it can make our Mac up to 40% faster, it can be amazing. Installing OS X El Capitan 10.11 from scratch So sit back leisurely in front of your Mac and let's start making it perfect to enjoy the improved performance of El capitán. The process is very very simple and similar to the one we have detailed with previous versions of OS X although, yes, it will require a good deal of time. We start here a classic, how to install OS X El Capitan from scratch and thus eliminate from our Mac any trace of garbage and / or small errors that may have accumulated after successive updates.

Install os x el capitan 10.11 or later