Here’s a moment of relief for you on the 777 to 787 differences course. The alternate gear extension works exactly as on the 777 - the FCOM description is even word by word identical.
A dedicated DC powered pump supplies hydraulic pressure with fluid from the center system to extend the gear. Door and uplocks are released, and the gear free falls to the down and locked position.
The position of the regular gear handle doesn’t matter, but the non-normal checklist will have you select it down anyway.
A mistake sometimes seen in the simulator is to not hold the ALTN GEAR switch in the DOWN position for long enough. If you carefully read the checklist it says to keep it there until all gear indicate down or in transit. Do that.