You cannot and should not start removing the old windows till you have ordered the vinyl replacement windows. Here's how you go to measure the size of windows to be ordered.
We'll start with a single casement window, no fixed panels. On throwing the window open to the outside you will see a metal lip of about 1/2" width extending from your drywall on all four sides. This part of the frame remains in its place as the new window is going to fit inside these lips. For finding the required width, measure left to right from lip to lip, then subtract 1/4" to get the final figure for ordering. The same way measure the height. Suppose the width measured 35 3/8" and the height was 38 3/8", lip to lip. So the size of your new window would be 35 1/8" (35 3/8" -1/4") X 38 1/8". If you live out west, where retrofit frames are available, you may order the retrofit style frame. In parts of the country where only replacement style frames are available, you will be required to add trim to the outside after you have installed the window.
Once you have received the windows you can go ahead with removing the old window.
While removing any window or a part thereof, you must take the precautionary measures of wearing the gloves and safety glasses. Single casements without fixed panels are the easiest to be taken away. As you throw the window open, you will see two pivot assemblies. One on the top and the other is at the bottom. Cut off the metal piece where the pivot pin is attached. Proceed the same way till the entire window frame and glass is removed. Now, take out the crank assembly by unscrewing the screws that hold it in its place inside the house. And, finally remove the protruding metal studs that you cut at the pivot to remove the window. The easiest way of doing it will be to clamp a pair of channel lock pliers or vice grips as close to the frame as possible, then raise and lower the vice grips to break the metal off. And, it's done.
In case of an arrangement comprising of casement and fixed panels, you first take away the casements as already described. Then, go to remove the fixed glass. I would put duct tape all over the glass. Then, use one of the two-dollar glasscutters that are easily available at any hardware store. Lubricate the tip of the cutter with a drop of any oil before applying each cut. From inside, score the glass across all the four sides. Now tap the score all around the glass with the tip of a screwdriver. Put an old sheet down on the outside of the window.
Now you may knock out the glass at the score by tapping it with the handle of a hammer. You will see a vertical metal bar in the center where the casement window gets locked. Next, cut this bar where it meets the frame coming from the drywall with the help of a hacksaw or a reciprocating saw. Bits of glass will be protruding beyond the metal lips. You need to remove these, so they don't hinder the installation of a new window. The glazing putty could be left in place, since it will get concealed after you have installed the new window.
