I need to cut some grooves into a flat surface. I have watched tutorials with cylinders, where they extruded faces inward to create the required effect, but rectangles only have six faces, so extruding one of them would accomplish just as much as simply resizing the object. How do you divide a rectangle's face into a grid of extrude-able squares?
Select the face where you want to add the groove. Press I to inset a new face. Then scale it with S and constrain to a axis (for me it was the X axis), press E to extrude the groove.
Or if you want the groove running all the way across the side, add two loop cuts with CtrlR. Then push them closer, by scaling same as above. After you have it the size you want, select the two outside faces and E extrude them out.