The easiest method I have found and works for me is:
When the speaker first pulsates move the left stick left
and for the second beat move the left stick right
, etc ... While doing this, if you noticed the meter did not increase or if your controller vibrates, it means you are doing something wrong - Try to alternate your stick direction, eg instead of left-right-left, try left-right-right.
Keep following this pattern until it says HOLD. Stay and hold your left stick either to the left or right. A second or two later, you will see greyed out symbols that will say 'RT' or 'LT'. Alternate the left trigger
and right trigger
with the beat. Hit these triggers one after the other in time to the beat for a massive bonus. Do this successfully; and the whole dance sequence will be reduced massively.
Eventually the girl will dance with you, keep doing this and it should pop up.