Under the device selection choose the last option, Android *.*.* Any Build.
Root it with the custom recovery box checked
Now, if you want to return to 4.1.2 stock without root click the appropriate button on the root toolkit. Don't forget to relock, if it bothers you.
To continue rooted:
On the bottom left side is a button to LAUNCH advanced options, click that
Then on the top right click 'Flash Zip' and follow the instructions.
You want the stock 4.1.2 rooted ROM, not just the update from 4.1.1.
Any other questions should be asked in the root forum.
Once I had a Nokia 6015i, but it wasn't smart enough.
Then I had a Helio Ocean, but Helio went adrift.
Then I had a Palm Pre, but that ended PREmaturely.
Then I had an HTC Evo 4G, and we had a grand times!
Now I have an HTC Evo 4G LTE, and all is right with the world.