Before attaching the necks you must cut the notch for the neck to fit into. I use a jewelers saw and a sharp Exacto knife
I use an acrylic caul with two small holes drilled in it for tiny brads. Driven through the caul and top, they will index the top properly
Check and check again before you add the glue and clamp over the caul and neck. You must get this perfectly Straight. Aw! Perfect!
It is vitally important to build a straight instrument. Put a pencil mark on the center line and measure carefully.
I fix the ruler at the center with a clamp, now it swings readily to line up the top mark and the straight line drawn on the neck
this is what it looks like from behind. The sides will come around the curve inside the pencil line and enter the slots in the neck.
And here they are a few hours later, The glue is dry and the necks are attached straight and true. I must be time to start bending some sides. Oh, I must now glue on the Butt blocks.