After you get a few estimates for new uPVC Windows you may decide to save some money by installing them yourself, a little diy windows job, so we have written up this set of steps on how to install replacement windows, and a sash window in particular.
When you start to install your replacement upvc sash window, first remove the old sash. A normal double-hung window consists of an inside stop to hold the bottom (inside) sash in place and a second (blind) stop between the two sashes. The stop on the outside is not removable.
Use your screwdriver or pry bar to gently remove the inside stops so that they can be reused. Remove the sash ropes and take out the lower sash, then remove the blind stop. Then take out the upper sash.
Put the new window in place to check for fit. It will have fit snugly against the stool (the flat piece) on the inside and the stops of the old top window on the outside. If there is a gap then the new window is narrower than the old one and the gap will have to be filled. To fill this gap you will have to nail a strip of wood to the jam along the outside stop.
if your sills are sloped, hopefully the replacement windows you are installing will have a sloped extrusion on the bottom to match, but if yours are flat you can use a shim to help support the front edge of the window, maybe a piece of lattice or door stop.
Or you could cut an angled support. To do this set the windows in place where they will be fitted and then measure the distance between the bottom edge of the window and the sill.
You can do this for all of the windows at once or one at a time as you need them. Plane down the edge of the strip at about the same angle as the sill. To install the strip just nail it to the sill at the point where it will be flush with the edge of the window (so that it sits on the strip).
Make sure that the window is centered in the opening; use a square to make sure that it is squared up properly, then use shims to make sure the window is secure in the opening, keeping an eye on it to make sure it is still centered. Make sure the shims go in behind the predrilled holes for the screws (so that the screw goes thru the shim). Then you can insert the screws into the side jam securing the window in place. Then reinstall the stop on the inside.
Check the window for smooth operation and that every thing is working as it should then caulk the outside edge of the window to seal it. Now you can step back and admire your new uPVC Windows, knowing that you did it yourself.