To strip the walls to studs, use a hammer and a pry bar to pound off the old plaster. Be careful of electrical wires and boxes.
Pry the lathe away from the studs and remove the old nails.
Save some of the old lath just incase you need to fur out the studs when putting up the new drywall later.
To remove the old window, remove the outside trim using a hammer and pry bar.
Remove the aluminum trim (Image 1) as well, and be careful not to damage the siding. Save pieces to reuse.
Also remove any trim material to expose the nails that hold the window in place.
Inside the house use the reciprocating saw to cut through the nails. Slide the blade through the gap (Image 2) and cut. Have a helper to make sure the window does not fall out.
Note: When you build a rough opening for a new window, make it big enough to accommodate the window and a false frame that will finish the rough opening. Then add 3/4" on all sides to allow some wiggle room to make the window plumb and level.
Remove the old sill using chisel and hammer.
Mark the center of the old sill so the new window will be centered.
Using a cordless impulse power nailer, re-nail the old sill to reinforce it.
Then nail down a new 2" x 2" to form the new sill.
Drag the center line down the new sill using a square, and use a tape measure to make the new markings for installing the new window.
Caulk the seam between the new and old window sill and the corners of the framing for added security.
Use a trim gun to nail in the false frame in place at the bottom and the stops.
Caulk and then it's time to place the new frame into the rough opening.
Note: You may need to use shims to level the window.