I did it to my old house windows.

Rack out the old putty (not it all, but enough to get a good bit of putty into take hold)
Using your fingers (easiest), roll the putty up in a sausage, then press in.
Using a wet knife (or finger), spread the putty to make sure that it sticks to the window and frame to that it is water tight.

Very easy, but boring after you've done 3 windows!