Ones complement
The ones complement of a binary number is obtained by flipping 1’s and 0’s. Therefore if you were to add a number with its one complement you would end up with a number of all 1’s.
One’s complement addition is handled just like normal but you carry ones that would overflow to the first position. i.e. 10 + 10 = 01.
One’s complement subtraction is handled similarly but you borrow from the end digit when required.