Doors and Windows - New casement window

I have a double hung crestline window in the kitchen, maybe 35 years old. the plastic jambliners are failing and generally it is poorly made anyway. I want to replace it with a new casement window. I have never done this type of work before but wonder if anyone can give me a clue of what to expect when I try this job. Maybe even a small cookbook list of how to.
thanks

You can order vinyl replacement casements to fit the old frame, if it is good condition(no rot). Installation is fairly easy for the average DIY skills. And a casement is a good choice for a kitchen, no bending over to lift the sash. Careful measurements are critical for a proper fit. Measure the actual frame, not the stop moldings, at top/middle/bottom, and all across top to bottom, check for square. Use the smallest measurement for your opening size.

In addition to the retrofit method that Just Bill mentioned, where a custom sized window is fit into the existing frame, you also have the option of completely tearing out the old window. You would then order a custom sized new construction window (with nailing flange) to fit into the existing rough opening, once the old window is removed. The exterior would be trimmed with brickmould or similar, and the interior would be finished with new interior trim.