For FCPX you just simply import the footage, put into the timeline & then select/highlight it.
Open the Inspector window & you should automatically be on the Video setting (as opposed to Info).
You'll see the Transform box - open it.
Then expand the Scale section & that will reveal x & y axis.
If you are using a x2 anamorphic you just simply change the y axis to 50% (this is the squash method & your footage will be 1920x540).
Stretch method (so your footage will be 3840x1080) see this tutorial (you'll need Motion):
For other anamorphics (squash method) with a different aspect ratio, you can simply do the Math (eg. x1.5 anamorphic - you divide 100% by 1.5, which gives you 66.67%).
Camera stuff - this all depends on the camera (Canon try different Picture Profiles & Magic Lantern / GH2 try different hacks).
But basically once you've attached the Anamorphic to the taking lens (& aligned it - just shine it towards a bright light & make sure the flares are horizontal) you're ready to go.