Making a two time border is really as simple as it looks. It just takes some time since you have to work in shifts, unless you are VERY good in cutting masking tape. I liked using a simplier method, with less room for errors, and thus do this in several steps.
1. Make sure your canvas is prepared with white gesso/primer. Whether you do this yourself or you buy it already primed does not matter. Be sure it is dry before you proceed.
2. Calculate the width of your borders. Do this by adjusting that width to the dimensions of the used canvas. My guess is that Bob uses 1 inch borders for his 18"x24" canvas. I took less since I used a 16"x20" canvas. My dimensions were 2 cms for the black border and 2 cms for the white. BE SURE to measure the black border distance FROM THE VERY TOP EDGE of the canvas since a canvas always bulbs a little in the greater center when stretched over the wood and that can easily be 2 to 4 mms wider then. You don't wanna calculate those extra mms in since your black border will then show too small compared to the white one.
3. Place masking tape along the LONG sides at the calculated distance. Make certain they are adhesive enough (old masking tape sticks less good!), follow a tight straight line with that tape and firmly stick it to the canvas. Paint both long borders with black gesso.
4. After drying (I roughly take 30 minutes or so), take off both tapes carefully and then place them over the SHORT sides of the canvas at the calculated distance (clean side of the tapes towards the inside!!). Or you can use two fresh ends of masking tape of course, whatever you desire. Take the same precautions and actions as in step 3 and paint both short borders with black gesso. Allow to dry for 30 minutes or so.
5. Take both pieces of masking tape off and throw them away. Then add fresh masking tape on all four sides on the calculated distance for creating the white border. Do this firmly by sliding your finger several times along the edges to really make it stick but avoid bending the canvas too much since that might loosen the tape again somewhat. MAKE CERTAIN you add extra paper or thin cardboard (e.g. picture) on all four sides to completely cover the black border you have created. Else, it will be spoiled with paint strokes and/or sprinkles when busy painting. You really wanna avoid that!
6. Make your painting and when finished, carefully tear of the masking tape and additional covering paper/cardboard. Do this either in parts or ask for a helping hand to assist you.
7. Enjoy your wonderful painting to the full!!