With thought control
Per this answer at Science Fiction & Fantasy
Tony Stark uses something called brain computer interface that translates brain EEG signals to control machines. So if Tony needs to move the suit's arm he just needs to think about moving it. This technology currently is very slow and not advanced as Tony has but it might get to that level in future. The electrodes of the EEGs are shown in Avengers and Iron Man 3 many times.
Since then advances in Iron Man armour have removed the need for external electrodes and they are, presumably, nano sized now as per the rest of the armour and enhanced, with AI compatability.