Measure the width of the window or door opening or the distance between the outside edges of the opening's casing. Measure the width of the valance's mounting board. Subtract the opening's or casing's width from the width of the mounting board. Divide the difference by two. The result is the distance that the mounting board must extend beyond each side of the opening to achieve a centered installation.
Align a straightedge with the outside edges of the opening or casing. Butt the straightedge's top end against the ceiling. Use the straightedge and a pencil to transcribe the location of the opening's edges onto the ceiling with a pencil. Measure away from the edge marks with a tape measure. Mark the distance that the board must extend beyond the edges on both sides of the opening with a pencil.
Pierce through the valance's fabric at each predrilled screw hole with a utility knife. Align the valance's ends with the outer marks on the ceiling. Butt the rear edge of the valance against the wall and the top face of the valance flush against the ceiling. Press a pencil through the screw holes and against the ceiling to transcribe the locations of the holes onto the ceiling's surface. Remove the valance from the ceiling.
Get a twist bit suitable for predrilling molly bolt holes. Refer to your bolt manufacturer's specifications for precise drill bit diameters. Mount the bit to the power drill. Predrill bolt holes through the screw hole marks on the ceiling. Remove the bolts from the molly bolt assemblies. Slip the bolts through the screw holes at the underside of the valance's mounting board and thread the bolts into their toggles.
Lift the mounting board to the ceiling. Align the toggles with the predrilled bolt holes. Fold the toggles, one at a time, and push them through the predrilled holes until their legs spread and grip the interior face of the ceiling cavity. Twist the bolts clockwise from the underside of the mounting board with a screwdriver. Tighten the bolts until they pull the mounting board and valance flush against the ceiling's surface.