I've used 6mm steel plate, I marked the plate out and then centre punched it for drilling.
Step 1: Drilling the Dowel Holes to Size
Starting with a 3mm drill bit I worked up to 6mm to 14mm range of holes for various dowel sizes
Step 2: Cutting the Serrated Holes With My Jigsaw Using a Fine Metal Blade
Using my jigsaw to create the serrated holes, I ground the blade for the smaller holes
Step 3: Preparing the Sticks
I wanted to use square sticks in the plate to take the dowel down to the size.I inserted the stick into the dowel hole and turned it by hand to create the round so it would fit in the drill chuck
Step 4: Using the Dowel Plate
The holes are 1mm in size difference I started at 14mm and went down to 6mm with the same dowel but normally i would prepare the dowel stick to the correct size for the dowel required.