To do this, you can follow the steps below and/or watch the video.
The first step is to make a Mac OS bootable USB drive which is outside the scope of this blog but easy to find with a Google Search.
The next step is to boot the bootable drive. Insert it into a USB port, reboot your Mac, and hold down the Option (ALT) key on start choosing the USB installer disk.
The next step assumes you are willing to erase the Mac completely assuming you have it all backed up on Apple's time machine or a similar program. Once the installer loads, choose the Utiliies -> Disk Utility option. From there we'll setup encryption, erase the disk and choose the encrypted journaled option. Pick the password you want for your drive and select Erase and remember the password chosen here.
After it is setup with the encryption key, begin the installation process.
The Mac will reboot after installation. If all went well, you would be asked for the password you chose above. Enter your disk password at the password entry prompt and the Mac should begin booting up. You must go through the configuration steps and may optionally recover from your Time Machine drive when prompted or start over with a blank install.
And that is all there is to it. It is pretty much as easy as using File Vault normally.