D-Max or "Maximum Density" testing is a means of identifying the shortest exposure time that will produce the maximum tonal value for a printing process.
Take a stip of clear negative medium (This could be a blank strip of base film or a piece of transparency you're using to print on for a digital negative) and place it on top of your printing medium (silver gelatin paper, coated cyanotype, etc.). Cover up all but about 1" of the paper, then progressively expose more and more of the paper (some folks do 2 seconds, 4 seconds, whatever). Develop the paper and observe! Moving from the lightest strop to the darkest, at some point you should observe a strip that is no darker than the previous one. This is your d-max! To find the d-max time just add up all the strips and multiply by the time. Magic. If you don't find a strip that isn't darker, then there's more density to be found. Repeat the expirement but start where your last strip left off.
Taking a step wedge like the Stouffer 31 step wedge you can simplify this process. Instead of making test strips one at a time, the wedge can do the same work all at once. In this method you just put the step wedge on top of the paper and blast it with at least enough light to get to d-max (if you don't know what this would be take a stab and adjust).
Once you have the developed paper you can look at the step wedge. Like before find the first step that isn't any darker than the previoius strip. Count the number of steps from that step to the first (darkest step). We want a minimum dev time, so we want that "first not darkest" to be on step 1. To do that we would reduce our exposure time by the number of steps we counted.
For a 31 step wedge each 3 steps represets a stop so we end up with needing to take the cube root of 1/2 (a stop) take that value and raise it to the count and multiply by our exposure time.
A 21 step wedge is the square root of 1/2.