I think you should try to do it without taking the heads off or it will never get done
so:
1. pop off the oilpan.
2. remove the big end caps on the two cylinders you want to deactivate.
3. pull the sparkplugs on those cylinders
4. take a washer and a long drywall screw and drill it into the piston through the sparkplug hole to "secure" it at the top of its travel.
5. sawzall off the bottom end of those con rods so the crank doesn't hit it (perhaps do this earlier so you don't stress out your drywall screw).
6. remove lifters on those cylinders
7. enjoy 2 cylinder goodness