When you're making a planter bed for vegetables, you want a depth of at least 12" because the bulbs need room. FYI: carrots need a little more depth, so keep that in mind if you're planting them.
The cheapest, most durable wood is treated lumber, but if you're planting vegetables we strongly recommend using cedar. There are toxins in treated wood, namely copper, that can possibly leach into the soil. This isn't supposed to happen, but it isn't 100% reliable, so we opted for (and suggest) all cedar planks. Juniper is another good option and holds up well under rainy weather, but because cedar is a little cheaper and measurements are more exact, we went with that. Because it varies a little in width and length, if you go with juniper we recommend that you ask to be able to pick it out yourself so that you can make sure you get exactly what you want.
Mise en place. That's a fancy French kitchen phrase for "everything in its place." Make it easy for yourself by keeping all your tools and materials handy.
Allow the countersink bit to change your life. Measure and mark where to drill the pilot holes on each plank, and then countersink your little heart out!
Now it's time to grab your drill and set the first side, attach ends with large screws. Attach ends on second side with large screws.
Square and level first side. Drive stakes in corners. Attach stakes to boards with small screws. Rinse, lather, repeat. Do this with the second side.
Drive more stakes all along the inside of the box, leaving about 4 inches of stake sticking above the board. Attach stakes to boards with small screws.
Attach upper layer of boards to themselves as you did on the bottom layer, then attach boards to stakes. Oh, and admire your handiwork!