"Cradle" Joint

Start with a workpeice that is flat, square, and true! Use a SHARP, FINE pencil and an accurate square. Carefully mark the dimensions of the pocket. 

To help the saw get started accurately, use a sharp chisel to cut a triangular chip at the corner of each face. Hold the work in a vise. Align the flat back of the chisel with the plane you intend to cut to make a shallow slice, then relieve the chip to create a small notch to guide the saw. 

Secure the work in a vise. Align the saw blade, your body, and your eye line so that you can follow the layout lines on both the top and face of the work. Hold the saw lightly (don't push down) and endeavor to use the whole length of the blade. Let the saw's own weight drop it through the cut. Saw as deep as you can before overshooting the lines. Repeat for each corner. 

To remove the remaining waste, systematically chop across the long grain fibers, working your way down gradually. Clamp your workpiece to the bench so it is well supported. Orient the chisel so it runs perpendicular to the long grain fibers. Hold it like a pencil, with your hand registered against the work's surface. Chop down by striking the chisel with a wooden mallet--one firm hit. Shift the chisel and chop again. Repeat across the whole pocket, staying about 1/8" away from the layout lines. Clear the chopped surface, using the chisel with its flat face down. Repeat this pattern, chopping and clearing, until you get to final depth. You should see three clean, sawn, triangular faces from your initial saw cuts. 

Working to the pencil lines, carefully pare the walls clean. Use the widest chisle that will fit. Reference the flat back of the chisel to the sawed triangles. Take light cuts. On the long-grain faces, work primarily across the grain using hand-pressure. On the end-grain face, chop with a mallet--taking care to keep the flat back of the chisel perfectly vertical. It may help to clamp a guide block with a flat, squared face along the layout line to help guide the chisel. Take care to clear crumbs out of the corners. Avoid using the chisel as a lever--the goal is to *cut* with the edge rather than split with the wedge.  

Use a combination square to check that the walls are square. If the walls are angled out, pare them to square. If the walls are angled in, you might choose to glue in a slip of wood and re-pare. Check the fit with the other member of the joint. If there are gaps, go back to the combo square to identify and correct the source of the error.