Jeff, there are two methods of constructing draw-leaf mechanisms; taper up bearers and taper down bearers.
Taper up bearers are tapered at their outer upper ends with the draw leaves attached to the tapered edges of the bearers. The bearers run through cut-outs in the end frames and down under a central, horizontal member which is affixed to the top of the table's side rails. There is no clearance between leaves and the main table top and old table leaves usually show the scars of being dragged out and pushed back in. The cut line for the taper is an extension of the underside of the central top (with the bearers in the withdrawn position). Early tables were utilitarian and didn't posses delicate patinated or polished surfaces.
To protect the surfaces of the leaves from scratches, it's possible to glue strips of baize (not felt!) to the underside of the central top. If doing this, make sure to cut recesses into the underside of the top for the ends of the baize strips to reside in. If you don't, chances are that some day when a leaf is being pushed back in, it will catch the end of the baize and mush it between the leaves.
Taper down bearers are perfectly flat on top and horizontal, the requisite taper being on the outer bottom edges. Again, the draw leaves are attached to the ends of the bearers and the bearers run through cut-outs in the end frames, the tapers forcing the leaves up as they are withdrawn. The inner ends of the bearers in this instance run directly against the underside of the central top. They also run through a vertical central support with similar cut-outs to the end frames. The central support is either attached to the side rails via sliding dovetails or multiple through, or blind tennons. The cut line for the taper is (lets say for the RH leaf) a line extending from the left side of the central support to about an inch beyond the outer face of the RH end rail (or how ever far out you desire the bearer to extend (an inch is a good figure for a table with the top extending 4" over the frame) and vertically, by the thickness of the leaf/top.
Only with the taper down bearers can the central top be caused to lift slightly to clear the leaves. This is achieved by ensuring the taper extends just beyond the central support. To make the centre top drop back down to the correct height once the leaf has been withdrawn, secondary, short, shallow tapers must be cut into the underside of the inner end of the bearers. Obviously the central top must be allowed to rise and fall without altering its relative position on the table frame. This is achieved by gluing blocks to the underside of the top, either side of the central support.
In both cases, pegs are normally inserted (often dry) into the sides of the bearers which stop against the inside faces of the end frames to prevent the leaves being withdrawn too far.
