Step 1: unlock the bootloader
Step 2: Download the TWRP image to your PC (place in same folder you have your fastboot and adb files), Download the SuperSU.zip and place on the Nexus 7's internal memory
Step 3: Boot Nexus 7 into bootloader (from powered off hold power+volumeup+volumedown)
Step 4: Open command line from the folder where you have the TWRP image (easiest way is to navigate to that folder, shift+rightclick, open command line here)
Step 5: type command "fastboot flash recovery filename.img"
Step 6: reboot into recovery by entering the command "adb reboot recovery"
Step 7: Flash the SuperSu
Step 8: Profit!
Via Android Police