Writing a letter and going through congress and the senate, usually signed off by the president.
The significance of Christ's presence during the crowning of emperors in Byzantine art relates to the notion that this individuals are pre-ordained by god with certain powers and thus are ordained to be leaders of their community. By placing Jesus in this pictures it signals that these emperors were chosen by god and Jesus to rule thus granting them religious legitimacy to their rule.
FDR had tried to pass legislation that would let him add up to six more justices. This was because the court had struck down some of his New Deal legislation and so he wanted more power. As shown though with the theory of separation of powers, it would have to go through Congress, who refused to allow the clause that let him "pack the courts" and as a result only reform for lower courts was passed and FDR failed in his main goals, but over his twelve years still appointed most of the justices that were on the court by the time his tenure as president ended.