Step 1: Tools and Equipment.
Hold your sander in your dominant hand and with your other hand in a glove hold the wheel from spinning, start sanding each knob down to where it is almost even with the rest of the tire, working your way around the tire on one side. Once you have them all evenly close to the smooth portion, release a bit of pressure with your glove hand and allow the wheel to spin as you are sanding, you will find you can adjust the speed of the spin by varying the angle of the sanding disc and what portion of the disc touches the tire, with a bit of practice you will end up with something like the tires pictured here.
Step 3: Repeat for the Other Side.
The next step will give some pointers on which tires are best for making slicks and why.
Step 4: Which Tire Is Best?
The three matching tires I had were all the same brand but 1 was brand new, never used and never been in the sun, 1 was slightly used but left in the sun for a year, and the last one was practically new but left in the sun for approximately four years.
The results were interesting considering these were all the same brand and model tire, the older and harder the tire was, the better the end result! The brand new one flexed it's rubber knobs so much as it was being sanded that it sanded through in places as you can see in the main photo for this step, the slightly older one was nice and even but ended up weighing less than the oldest one, indicating that more rubber was taken off it to achieve the smoothness required.
After doing those I went looking for the oldest and heaviest tire I had, which was the Kenda Comp pattern tire in the other 2 photos on this page, it was approximately 10 years old and had been left in the sun for most of it's life, the bike it came off was found at the back of a scrap metal yard where it had been for many years, It had no cracking on it but was really hard, the results of this tire were the best of all, really smooth and the most rubber thickness left when finished.
So the best tire to do this with is one that is harder, and possibly older...
Step 5: Clean Up!
If you like this Instructable why not check out my other one here: https://www.instructables.com/id/Making-wide-bike-wheel-hubs-and-joining-them-to-ca/
The last 3 photos are from that Instructable...
Have fun and be safe!