Building a small tool cabinet video
In the video he shows making the sliding dovetail but when cutting the dove tail into the sides he says cut the 5/16 dado and then come back using the dovetail bit. But the piece going into the dovetail is a whole lot wider? Is a step missing for creating the proper width of the dado??
Without knowing which video you are referring to:
Because you cannot sneak up on the depth of a dovetail you have to start the bit at full desired depth. Pushing a dovetail bit through stock for a sliding DT alone would pack up with dust and burn up the bit (good luck backing out of a stopped cut!). The dado, either with a router or on the tablesaw opens an escape channel for dust and sets you up for a one-sided cut. The width is adjusted when you finalize the second side of the channel's dovetailed interior.
If you look at the plans, the top and bottom boards are 5/8 and the shelves are 5/16.