Do-it-yourselfers is always better to get a prehung interior French doors unit. This means that doors are already on hinges and is located in an inner frame (casing), so all you have to do is set up unit in opening at once. Opening should be 1/4 to 1/2 inch larger around than outside of casing
Set prehung interior French doors unit in boxed-in opening. Set your spirit along one inner edge of door casing (border of wood that surrounds two doors). Insert shims between casing and wall, while level readings at various points. Continue until unit is completely level and plumb.
Test both prehung interior french doors opening and closing. If they bind, release liners around sides. Open both doors. Use your screw gun sink 2-3 / 4-inch screws through shell to surrounding wall frame. Bury screw heads slightly into wood. Do not tighten them so much that it pulls apparatus on level. Removed middle hinge screw from each hinge on each door. Replace screws by 2-3 / 4-inch screws, so they go out back of house and embed into surrounding frame. Use your razor knife to cut off overhanging shims. Space between casing and wall will be hidden when you trim out door.