A step wedge is a series of exposures made a specific intervals of time. take a piece of enlarging paper and cover all but a 1/4 inch and make an exposure i.e. f8, 3 secs.
move the cardboard and make another exposure, continue to do this until you have exposed the entire piece of paper. process . what you will have is a step wedge moving from very little gray to black.

There are various methods to learn how to print and I do not want to get into a flaming war over which one, one should use. The question is how to find minimun time for max black and this will do the trick.
Find the point at which you can no longer see a line between shades and that is the minimum time for maxium black.