Summarize this article:

You can use new, clean, heat-treated pallets to make simple, neat raised beds. You can take the pallet apart to make a deep raised bed from the timber, or just lay one on the ground to make a shallow raised bed.  Keep in mind that you won't be able to raise the bed too high with just one pallet, but it's a good start. Raised bed gardening means you don't need to bend down as much to tend plants, need to dig less and it really helps improve drainage. Ideally, you should start by eliminating weeds underneath where your raised bed will go. Try chemical controls, or lay down some newspaper or cardboard.  You'll want to close up the edges of the pallet all round the narrow sides, to prevent weeds from poking through. To do this, you can nail thin planks around the 4 edges, or use strips of weed control fabric. Plant it up through the slats and water well. Because you are raising the bed very little, this method will work best for shallow-rooted plants such as lettuce and strawberry.

Summary:
Decide what kind of raised bed you want to achieve. Eliminate any weeds and close up the edges of the pallet. Lay the pallet on the ground and fill with compost.