One note to the adequate instructions others have given. Try and remove ALL of the roots and other plant material you can. Your work will be much improved. Leave in stones, gravel etc. One project was a bench type for a patio.
Pallets are available free from our local newspaper.
Seal the pallets with a wood sealer or stain. I have been investigating a number of options for a very large circular driveway. It had been a driveway many years ago and you can still find white quartz stone among the ground cover that has grown up there. I have been trying to limit the amount of space that requires mowing on my large suburban lot, since the driveway was relocated. My spouse and I did the newish drive in interlocking stone - a heck of a job.