It's not truly difficult, but it takes a bit to get it right, especially with how heavy the doors are.
To make things easier on you, you will need to remove the driver's fender so that you can get to the hinge bolts.
You will need to remove the striker pin out of the door as well so that you can set the door in place and be able to let it swing to test how well it shuts.
Next, look at your passenger door. If it's lines are nice and even, measure them and try and find some metal dowels (3 or 4) that are the same diameter as the gap on the passenger side bottom, and also for the gap by the striker side of the door.
Next loosen (NOT remove) the door hinges on the Driver's side.
Slide several dowels under the bottom of the door to assure that the gap is correct, then insert the dowels by where the striker is.
Once you are happy with the gaps, tighten the hinges back up. Remove the dowels, and test the shutting action on the door.
Once you are happy with that, re-attach the striker pin, and check its alignment.
Next replace the Driver's Fender.