You would order your Self Tapping Screws to fit an 32 1/4" opening thus allowing the window to slide into the opening. The same idea works for the height as well. Measure the left side and right sides of your opening from the high point on the sill to (where the bottom sash sits when closed) and the top (where the top sash sits when closed).Take the smaller measurement of the two and that is your height Number the window openings as you measure them so if there are different sizes, you will know where they go when you get them. With all the measurements is hand, your off to the store. Once you have picked the style, color and price you want, the salesman will take the order. HINT: Ask the salesman for a delivery date. Try to get him/her to put in down on the receipt so you can keep track of the order. Depending on the time of year, windows can take weeks to arrive. No one will guarantee a date but this gives you some idea of when to start checking on them. Check the order form very carefully before you order or give a deposit check. Once ordered, they cannot be changed. Look for quantity, color, sizes and style. If your existing home has what are called 6 over 1 windows (6 small panes in upper sash, one large pane in lower sash), you may want to match that style. Windows do come in 1 over 1, 4 over 1, 4 over 2, 4 over 4, 1 over 1, you get the idea. There are countless combinations to choose from. I ordered several windows for a customer once and specified 2 over 2 style. The windows came with the muttin bar (the piece that separates the panes of glass) in a horizontal position not vertical. I had never seen that before but since I had not specified it exactly, the company would not take them back. Whatever type you order, make sure your order slip says that is what you are getting. Now sit back and wait for the windows.
PREPARATION-Tools required-Measuring tape, pencil, 2' and 4' levels if possible, razor knife, pry bar or "cats paw'", caulk gun, Phillips screwdriver, paper towels and nail punch. SAFETY GLASSES AND WORK GLOVES! Materials required-Windows, 4,6 and 8 penny bright finish nails, small tube wood putty, approximately 2 tubes of high grade exterior caulk in a color to match your window per window (depending on size), small Phillips wood screws, loose fiberglass insulation.
Remove all interior window blinds, shades and curtains and put to the side. Very carefully, using your razor knife, cut along the interior window trim and the wall if necessary and along the trims on the inside of the window frame. This will help prevent cracking the wood when you remove it. You are going to re-install these trims. Carefully and slowly, pry the trims loose watching for concealed nail heads (especially common style with a big head) that will crack your trims if you pry too hard. Most of the time the finish nails will simply pull through the trim and you can remove them later. Lay each piece of trim directly to the side of where you removed it. You cannot use it elsewhere. It won't fit! Remove or pull all the nails out at this point in time. We don't want a nail injury from a rusty protruding nail. With all the trims removed, look in the bottom corner of the frame and you will find a small wooden "door" held in place with a screw. The screw may be covered with paint so you may have to dig around a little. Doors are usually 6-8" high so start there. Remove the screw allowing you to pry the door out and reveal the inside of the window frame. Now lift the lower sash all the way up. Look inside the door, you should be able to see the end of the sash weight. If not, don't worry, the sash cord is just short.
Now using your pry bar, remove the wooden parting bead holding the sash in place. It runs the entire height of the opening and looks about 1/2" square. You will more than likely break it into a million pieces. You are not going to use this again anyway. Once one side is out, the sash will slide out of the opening. Hang onto it! Carefully remove the sash cord embedded in the pocket on top of the sash. The rope may be held in place with a small nail. Just remove it. You can let go of the cord as the knot will stop it at the pulley. Lay the sash to the side. It is junk. Now remove the second parting bead. Again, carefully pull the sash cord in far enough to cut the knot from the rope. When you let go this time the sash weight will crash to the bottom of the little door you opened. Reach in and remove it. Also junk. Continue until you have removed the inside parting beads (do not remove the outside two as the new window will rest against them as a stop), four sash weights, four pulleys, etc. leaving you a clean opening. Scrape any loose paint or gunk built up in the openings. Here's where you get to install some of that loose insulation. Stuff the pockets and any openings in the frames. The use of the expanding foam products is possible but be very careful. It does have the ability to actually bend or bow the Concrete Screws when it expands. There are now non-expanding types which you could also try. The goal is to leave no openings where outside air can infiltrate into your home causing drafts and loss of heat!