Here is a great video for reference that I use. Part 1 is the E-step calibration Part 2 is that flow rate calibration I mentioned.
From my understanding the E-steps correlates to the number of microsteps of the motor to millimeters traveled. You can find what your current E-steps value is by loading up Pronterface and inputting certain commands.
If you are using the Lulzbot Cura program, Pronterface is the window that opens up when you want to print. The window where you can manually move the X,Y,Z and extrusion as well as setting the temperatures.
There is a command box to the right where you can input the M-codes and G-codes. Here is the list of the different commands. http://reprap.org/wiki/G-code
To find out your current E-steps value, you want to enter “M501” (cap sensitive). This will bring up your current settings on your machine. The M92 line is where you want to look. On the M92 line you will have an “E833” value of some sort. That is your current E-steps (I think default is 833 from my notes).
So after you go through the calibration process and you determine your new E-steps value, the way you input this value is on the same command line. I usually will type the following:
M501 (to see the current values)
M92 E800 (Change 800 to whatever your value is)
M500 (Saves your changes)
M501 (Verify the changes stayed)
I would then run through the calibration process again and make sure you end up with the 20mm±1mm left over.