The repair job doesn't actually require professional expertise, you can get it done yourself. All it requires is some basic tools and supplies, and a few hours of your time. The material can be bought from any local hardware store, and is not very expensive. One needs to understand how a window pane is fastened into the frame, before attempting to fix it.
With a screwdriver or a putty knife, scrape off the old putty from the window frame. It may require softening with a soldering iron. Small metal pieces, known as glazier's points, are driven into the frame underneath the putty to hold the glass pane in place. Remove these glazier's points by flipping them out with a chisel or pulling them out with pliers.
If you intend to replace the glass in an old wooden window frame, you need to apply a heavy coat of linseed oil to all the wooden sections around the frame. Allow it to soak in completely. The new putty that you will apply thereafter, will last longer if you let the wood saturate with oil.
Apply a very thin layer of putty around the frame where the new glass will be installed. It acts as a cushion for the glass to be inserted. The frame is now ready for the new glass to be set.
Take a roll of putty and apply it on the frame. Smoothen it out with a putty knife, scraping all the excess putty off.