How do I use a bootable disk drive to install Mac OS on an iMac 2011?
Note: The images shown below are from a VirtualBox install of El Capitan. The installation was performed using a physical USB El Capitan flash drive installer. The instructions for creating this flash drive can be found here. You should see similar images when you install to your 2011 Mac. 



Make sure the Mac is off.
Disconnect all devices from the Mac, except for any wired keyboard or mouse that you may be using.
Insert the El Capitan bootable flash drive installer into an USB port on the Mac.
Turn on the Mac and immediately hold down the option key until the Startup Manager icons appear.
Click on the text Install El Capitan below an icon. The Mac should start booting from the flash drive.
The first window viewed should be similar to the image shown bellow. Click on the Continue button.


Proceed until you reach a window similar to the image shown below. From here, select Disk Utility... from the Utilities pulldown on the menu bar.


Highlight the internal drive as shown in the image below, then click on the Erase button.


  Note: Your internal drive will not be named VBOX HARDDISK Media.



Enter a Name for the volume to hold El Capitan. Set Format and Scheme as shown in the image below. When finished, click  on the Erase button on the popup window.


When the popup window shown below appears, click on the Done button.


When you see window, similar to the image shown below, select Quit Disk Utility from the Disk Utility pulldown on the menu bar.


Highlight the volume you just named as shown below, then click on the Continue button.


Below is an image of El Capitan installing. To finish, answer any questions asked by the installer.