If you were to Google the term 'Window Lintel', you would find several hits on how to do this yourself. What most of these results dont' tell you is that a lintel is a beam put across an opening for a door or window in a wall to support the wall above it.
ARTEX Construction & Restoration, LLC knows that there are many 'Do-It-Yourself' projects out there that you can indeed do yourself. However, we suggest that projects like this, one that invlove the wieght of the wall and anything above it, is best left to window lintel contractors who know what they're doing. We use them in many types of wall construction, most typically in masonry such as brick, stone or concrete block.
ARTEX can also take care of removing and installing new window sills. This flat board at the opening of the window can become worn and weak, especially if the windows allows for seepage from the outside.