To change sequentially from first gear to sixth gear on a Brompton six speed bike, the gear levers must be in the following combinations; 

First gear- The left hand gear lever should be in the lower position and the right hand gear lever should be in the lower position.

Second gear- The left hand gear lever should be in the higher position and the right hand gear lever should be in the lower position.

Third gear- The left hand gear lever should be in the lower position and the right hand gear lever should be in the middle position.

Fourth gear- The left hand gear lever should be in the higher position and the right hand gear lever should be in the middle position.

Fifth gear- The left hand gear lever should be in the lower position and the right hand gear lever should be in the higher position.

Sixth gear- The left hand gear lever should be in the higher position and the right hand gear lever should be in the higher position.  

Although it looks very complicated when written down, when in use it becomes second nature. 

Another way to think about it is that there is a high and low option (left hand gear lever) for each of the three hub gears (right hand gear lever).