Do you want to change the momentum of something? Easy, arrange for a force to act on the thing for a while. Force accumulates momentum.
A larger force, or a longer time, or both, results in a greater change in momentum. Force and momentum are vectors, so both direction and magnitude are essential.
Add this change in momentum to an existing momentum, to calculate the final momentum. Add tip to tail, because momentum is a vector.
You can see the same accumulation of momentum by force, step by step. Each step is the same interval of time, so now you can only set the force. The momentum accumulates as time ticks on.