Lift the existing sliding doors free and remove them. Unscrew the track for the sliding doors, or cover it with wood trim if it's recessed. Skip this step if you're installing double doors in a new closet, rather than replacing an old door.
Measure the door aperture and purchase a pair of double bi-fold doors to fit. If you can't find an exact fit, buy a set slightly too large and shave it to size with a power planer.
Screw the surface mount track to the top of the frame. Use a plumb bob to locate the correct position of the lower pivot points, directly beneath the track. Screw the pivot brackets into place, one in either corner of the floor.
Set one of the doors into the appropriate lower pivot point. Snap the outer upper corner into the upper pivot, and the inner corner into the track. Repeat for the other door.