Remove the sliding doors from their tracks. Pry loose the tracking at the floor level with a small prybar, taking care not to injure the flooring material. Take screws out with an electric screwdriver in reverse mode. Install wood filler or epoxy to cover any holes in the door area threshold.
Measure the closet door opening to figure out how many French doors to install. Add a single set of French doors with one fixed, single French door, for example, across the closet opening. Install two French door units, side by side, if you have ample room.
Build any needed door framing to fit the French doors. Cut high-quality wood with a circular saw, for example, to expand the header or jack studs to make the opening fit the new doors. Add wooden shims under the wood framing around the opening to make it level and plumb. Check all surfaces with a carpenter's level.
Install and level all framing and door jamb materials before adding a metal door threshold. Cut the threshold material to fit the door width with a hacksaw or jigsaw equipped with a metal cutting blade. Secure the metal threshold piece into floor joists with 2-inch metal screws.
Get a friend to help level each door section. Drill pilot holes with an electric drill to hold the unit in place. Drive nails to secure it temporarily. Screw the door unit into place, following manufacturer's directions for exact diameter and length of screws for each section.