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This beautiful crate was made out of old pallets. If you have the right tools, and a lot of time, this is a simple project.
First, take a circular saw and cut the thinner slats off of the 2x4 supports. Cut beside the 2x4s and not into them to avoid nails. Once you have cut enough pieces off, move to the next step.
A large planer is best for the this part, but a belt sander with a heavy grit belt on it (80 grit or lower) also works. What you want to do is plane/sand the boards down on the two largest sides until you can see the grain and the boards are cleaned up. They don't have to be the exact same thickness but must be close. Also note that the boards will be sanded again with a finer grit to finish. Next, take a table saw and run the boards through to clean up the sides. Finally, take a miter saw and square off the ends.
Here is the part I can't really help you with much: the design. Remember that you want your project to be structurally sound. Other than that, you can pick from tons of options and sizes. Plan carefully, because even with a lot of planning, you can still make mistakes. To fasten, I suggest to use Titebond wood glue and a nail gun. Screws also work, but I still suggest glue. Something many people don't know is that once glue sets, it is stronger than nails! Hand driven nails are not a good alternative because of the possibility of missing and damaging the wood. Also remember to make sure your fasteners are the right size. Wood glue will freeze and this depletes the effectiveness. Store your wood glue inside during cold weather and take your project inside to dry, too. Once you have fastened the wood together, take a wet rag and wipe the glue off because it is hard to sand, especially in corners.
Now comes the final sanding. Take 120 grit or higher on a finish sander and sand any uneven places or glue or obscenities on the wood. If needed come back and sand by hand with the same grit. If everything looks good, it is time to stain/oil the project. A stain colors/brings out the grain of the wood. Oil protects the wood. My suggestion, especially for oak, is Danish Oil. It is a combination of the two. It provides a light coloration and seals the wood at the same time, where usually you have to stain, dry, stain, dry, oil, dry, oil, dry; at least. Once the Danish oil dries, you are finished.
It takes a lot of work, but it leads to a beautiful product. Hope I helped. I wish you good luck in your DIY'ing.