Extend a Windowsill
Select plywood the same thickness as sill; cut it as wide as sill and 12 inches deep. Attach bottom of shelf to bottom of sill with 3 hinges: 1 at the center, the others near the sides.

Cut 2 right triangles from wood thin enough to lie flat beneath folded shelf; trim tips. Brackets should extend 3/4 of the way across shelf and 3/4 down apron. Hinge brackets to apron. Paint before installing.

Convert a Cabinet
Use a square space, at least 14 by 14, deep enough to hold bottles on their sides. Cut two 1-by-12 boards to match dimensions of depth and diagonal between corners of shelf; sand to fit.

Notch middle of each piece at 90 degrees: With pencil and speed square, mark wood; jigsaw to make a 3/4-inch-wide notch that stops at center. Bisect pieces at notched points. Paint to match shelves.