[header] How to grow portobello mushrooms [title] Build a tray frame out of scrap wood. [step] Pick up some scrap wood from a lumber yard or purchase boards from a home improvement store. The frame should be about 8 in (20 cm) deep and at least 6 in (15 cm) long. 
Nail the wood together to form the bed. [substeps] Instead of assembling a tray, purchase a mushroom-growing kit from a gardening store or online.