Each replacement window company seems to have some variations in their replacement metrology, and therefore most (all that Iíve seen) provide specific instructions as to their installation. You should be able to pick up these instructions at local building supply stores that sell replacement windows. Pick them up BEFORE you start ordering windows, so you can ascertain the difficulty. Also, Iíve searched the web and found multiple sites that manufacture replacement windows. One of them may have online instructions to give you an idea.
Generally, the procedure for their replacement is straightforward, and will involve removal of the upper and lower sashes, including the inner and center guide strips. Assuming your old windows are made of wood, these guide strips usually pry out with the rest of the trim. If they are made of aluminum, different procedures will exist. Again, the instructions will cover all of that in detail. What you will have left is the original window frame, sans sashes, inner, and center guides. The outside guide remains, and is used to hold the replacement window in place.
Now, the replacement window is designed to fit within the existing frame, as the new frame is much too flimsy to do the work all by itself. This is one of the difficulty areas you will encounter. Although the basic replacement procedure it relatively simple in concept, the reality is that the old frame, which you now need to support the replacement windows, may be rotted, termite infested, or loose, or water may have been leaking inside the walls for years, causing structural problems which can lead to much more work than you planned for, maybe even more than you can handle. Usually you can predict trouble by a good inspection of the frames before you start, but there is always the possibility of the unknown.
The other difficulty area is the new windows themselves. Since they are designed to fit within existing frames, each window has to be custom-made for the frame it is going to fit into, so your measurements for the window order have to be exact. I prefer, when possible, to have the company or designated representative do the measuring even when Iím doing the windows, so that if there is a problem with the order, I am not on the hook to pay for another window.
I just wanted to point out the obstacles. Window replacement can be tricky, but not dangerous. Assuming everything in your house was built properly, your efforts should not cause the house to fall down. The biggest problems you will face will be getting the new window aligned and square so that it closes and seals right, and completing the ascetics of the inside and outside so the project looks finished. Usually, since the outside of the completed job will still show part of the old window frame, vinyl or aluminum capping is in order. Of course, every project has its ďhiddenĒ problems waiting to be discovered, but you simply take them as they arise. Just take your time, do one window at a time, and donít be afraid of admitting when you get in over your head, and chances are something wonít happen thatís irreversible. In fact, if you determine professional help is in order, each step you do yourself will make the completion less expensive. Just keep a sheet of plywood around so that you are not stuck with an open hole in your house should you require more think time than available daylight. And donít be afraid to talk to local experts on the subject. Their knowledge can be invaluable.