If you have a blank wall with a window, the simplest way is to flank the opening with ready-made shelving or wardrobe units and span the distance with a seat. The trick is getting the measurements right. Standard shelf depth is 12 or 16 inches; for comfort, the window seat should stand proud, for a total depth of about 18 inches. Wardrobe units for hanging clothes are about 2 feet deep, so here it's best to recess the seat by 6 inches or so. That way, you can sit facing into the room with your back supported and both feet on the floor.