Prior to installing interior French doors, take out the old frame using a hammer and pry bar. Being careful not to damage the trim of the window, take it apart in pieces, and remove it from the window opening. Mark the parts of the trim to remember how to put them back. Before beginning the installation, use a level and carpenter's square to ensure the window opening is completely even and makes a rectangle.
Place the new window into the opening, and secure it with shims. Try opening and closing the doors. They should move without any difficulties. If they work well, secure the frame with screws, put the doors in place, and install all the doorknobs and latches that come with the French door. Reassemble the trim once the door is secured in place permanently.
Before installing a French door inside your house, make sure you have a power drill, saws, screwdrivers, hammers and mallets. You also need screws, shims, a pry bar, a chisel and a tape measure. In addition, purchase a French door kit that includes all the necessary hardware. The installation may be a dangerous process, so always put on safety glasses before conducting the procedures.