Thanks. This cabinet has served me fairly well over the years, but I'm just a hobby grower.
So firstly, I germinate the seed using the paper towel method. Once it has a decent-sized tap root (about 1 cm long), the seed goes into a small rockwool cube. It usually takes a couple more days for the seedling to sprout up through the cube, so until that happens, I just leave the small cube (without burying it under the pebbles) and hand water it until the seedling sprouts. When it does, I put the small rockwool cube into a large cube, and bury it under the clay pebbles and turn the lights on.
The pump and feeding solution are contained in the tub below the tray. Check the Maui Waui link in my signature and you'll see how I set up my box. It's a pretty simply flood & drain system, with a 16L tank below the grow cabinet, and a water pump which floods the grey tray for 15 mins, three times a day during lights on, then drains back down into the tank.