The terms used to designate cousins—2nd cousins, 3rd cousins, once removed, twice removed, etc.—are totally baffling to most people, and with good reason. Generations ago, people stayed put, and half the town would have been related somehow. It made sense to keep track of kinship that no longer seems important.
The designation of cousins is fairly simple by following only two principles:
First cousins have a grandparent in common. Second cousins have a great grandparent in common, and so on. (Normally it would be a set of grandparents, both grandfather and grandmother, but we have to allow for modern marriages.)
“Removed” refers to cousins in different generations, so your father’s cousin, for example, is your cousin, too, but once removed. Looking from the other direction, the children of your cousin are also your cousins, once removed. Combining the two rules, your father’s second cousin would be your second cousin, once removed.
By applying just these two rules, you can figure out any distant kinship, but be aware, complex intermarriage can produce more than one relationship.