When you have Advantage on a d20 roll, roll a second d20 and use the higher of the two rolls. Disadvantage works the same way, but you use the lower roll instead.
You only ever roll one additional d20, even when multiple situations affecting the roll grant Advantage or impose Disadvantage.
If circumstances cause a roll to have both Advantage and Disadvantage, you are considered to have neither, and you roll one d20.