To find the relationship between two persons in the same descendancy, that is, they do have a common ancestor (actually, an ancestor couple) - find that common ancestor, by stepping back through the generations. Each do that. Plot one along the top, plot the other down the left border. From those end point, go down the chart and across the chart to where the lines meet.
That is your relationship. If the one on the top is a 4th Great Grand-daughter, and the one on the side is a 6th Great Grand-son, they are, therefore, 5th Cousins, twice removed (2 generations seperation).