Installing Slip-In Replacement Windows

I've recently been getting estimates on replacing old wooden windows in one room in my house with fiberglass replacement windows. The original windows aren't in great shape and probably are not worth fixing. I've discovered that window installers mark up the windows anywhere between 20% and 40% over the lumber yard for the exact same product, so I'm thinking about just doing this myself. The window frames are still in good condition, so I'm planning to install "slip-in" type retrofit windows.
I've never done this before, so I have a couple of questions: 1) Is installing these types of windows difficult, and what is involved? Do they generally mount into the frames with screws, or how do the window units attach to the frames? 2) I have two old wooden casement windows, which will be easy to remove because they are installed with hinges. But I also have one wooden fixed or "picture" window in the middle that needs to be removed. I have no idea how this sash is attached to the frame--with glue? How difficult is it going to be to get the fixed window out, and am I going to do damage to the frame trying to get it out?
Thanks in advance to anyone with any advice on this project.
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Thats why a pro is sometimes best. Go get a book
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Window installers get so much per window according to the size to be installed. If you mess up the window while installing it. there goes your warranty.
If they mess up the window it is out of their pocket.
Sometimes better to leave it to the professonals. I've seen what my son goes through to put a large picture window in a condo on the 10th floor.
PJ
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