Once the seed box/tray is built fill it to the rim with half screened compost or peat moss and half garden soil that you have screened to remove roots and rocks. Mound the dirt in the center, when it settles it will flatten out.
Sprinkle your seeds directly into the dirt and lightly cover with compost or peat moss. When the plants come up pick the strongest plants to plant in the garden. Some plants like mesculin greens can be harvested directly from the planter box.