Dear Student,
Force is a vector. So they add up by vector addition.
A. When the forces applied on the object are in the same direction. Then their magnitudes add up and the resultant force will have the same direction as that of the other two.
B. When the forces applied on the object are in the opposite direction and one of them is higher, then the magnitude of the resultant force will be the difference of the magnitudes of individual forces and the resultant force acts along the direction of the higher one.
Regards,