How does the president execute each of his roles? ?

The president executes each of his roles mostly by relying on the help of his administration.
President has the right to personally choose each head of government branches, which means that He could appoint the people that he could trust and has the capability to be in that position

The U.S. troops arrived just in time to turn the tide of the war in favor of the Allies. ... The influx of fresh troops helped to boost the morale of the Allies and played a major role in the defeat of the Germans. Wilson's Fourteen Points. After entering the war, President Wilson issued his famous Fourteen Points.