First off, if the studs are a bit off, use some shimming so that the new drywall goes on level. That is much easier than trying to level out a dip in the wall by skim coating.

For tips on taping and finishing, check out the site below.

Here is a link that might be useful: Drywall Taping Tutorial

This might be covered in the tips above, but I have found that using a light (we used one of those florescents made for under cabinets) to locate inperfections really helped.
Also, grit your teeth and keep at it. You really WOULD rather do one more coat and sand than stare at a noticable dip or other imperfection the rest of your time in that house.

Looking to skim coat the entire wall, not just tape the joints.

Need a hard, smooth surface on these walls. Last time I put up wallboard, I spent about 90% of the time shimming. Screwing wallboard on properly won't be a problem - getting it smooth might be.

This is exactly what I'm trying to do, but there must be some first time caveats. It looks too easy.

Here is a link that might be useful: finehomebuilding

OK, I'm confused. Are you removing the other 2 walls, or just looking to skim the 2 "new" walls and leave the existing 2 plaster walls alone? Also, are the 2 "new" walls completely rebuilt/reframed or are they just stripped of old plaster and lath?

If your "new" walls are actually the old walls just stripped they almost certainly aren't square, but as some one else said they're easy to fur out. Also remember that any electrical boxes will be set for plaster not sheetrock. There depths may or may not be a problem...again easy to fix.

Regarding the skim coat, don't use regular sheetrock. Blue board is the proper underlayment for a skim job. The paper's been treated to handle the moisture of plaster. If you really want to tackle this, your going to have to take the old Carnegie Hall joke approach...practice, practice, practice. I would build a mock up in the basement of a wall with blue board and get started. It's not brain surgery but it is technique. I'd rather waste $30 worth of material and a Saturday, than screw up a framed wall.

Shimming is very slow and a real PITA.
You can get a faster result by using 2x2s (I use ripped down 2x4s) and attaching them to the edge of the present studs protruding far enough to create a new flat plane for the drywall.
Find the highest spot on the existing studs and make the entire wall match that.
Either screws or a nail gun make quick work of attaching the new strips to the sides. Hammering nails allows the new studs to move around too much (and old wood can be very hard).
If the area is large enough a laser level projecting a plumb line and a wooden gauge can be used to quickly align the front edge of the new sisters as you move across the wall.
Attach new wood to every stud so the screw spacing will be uniform.
Some painters tape and a marker allow for transferring the stud locations to the floor for weasier wall fastening if there is any irregularity in the studs.

Skim cioating for smaller areas isfaster with a plasterer's trowel (instead of a srywall knife) and use Easysand or Durabond. 120 minute if you are a little slow, 90 if you are faster. If you mix these compounds about as thick as peanut butter (smooth with no lumps please) there wil be very little shrinkage as they harden. Unlike pre-mix that hardens by evaporation, Easysand and Durabind harden by chemical reaction.

If you;re looking to skim coat plaster, then reread spanky's post.

Blueboard followed by a single veneer coat of PLASTER.

No sanding, no three coats, just a single veneer skim coat.

I agree with...I think it was brickeye...about straightening the walls using 2-by's sistered to the existing studs.

Sister one stud plumb in one corner. Sister another in the opposite corner. Run two strings, one at the top and one at the bottom of the wall. Using straight studs, then sister to the remaining studs in the wall. When you set the string, use a small spacer, say 1/2" block, to hold the string 1/2" off the face of the corner studs. That'll prevent the interior studs from pushing the string around.

Plastering is a learned technique.

Best of luck with the project.

Mongo