I'll use myself and my distant cousin Nelson Drouillard in the example.
My Pedigree is: Mark10 Oswald9 Walter8 William7 Joseph6 Edouard5 Louis4 Simon Amable3 Jean Baptiste2 Simon1
Nelson's Pedigree is: Nelson9 Richard8 Felix7 Louis Denis6 Denis5 Louis4 Simon Amable3 Jean Baptiste2 Simon1
You can see that the common ancestor is Louis4
Put Louis4 in the "Common Ancestor Box"
Across the top boxes, fill in the Pedigree from Louis to Me
Down the left column of boxes, fill in the Pedigree from Louis to Nelson
Draw a line from Me, down the chart to the bottom
Draw a line from Nelson, across the chart to the right
Where these two lines intersect is our relationship
Nelson Drouillard and myself are 4thCousins, Once Removed