After you have dismantled all the pallets, gotten all the nails out... I would put the same depth boards in piles, so that when you are nailing ( I used an Air Nailer gun), use all out of certain piles so that you are scrambling looking for the right size, depth. Use a penny or a nickel in between each board.
If you click on "explore projects" then "build" then type "walls" into the search bar you can find many ideas. I found one for you, but there are many many more. There is even a "how to build a chicken coop" one that explains in detail how to use pallets for the main frames of walls which you could apply if that is what you are needing. Here is an idea I chose for you: https://www.hometalk.com/1998234/pallet-wall
Margaret, you will need to measure your work area carefully. Write it down. After collecting your pallets, take all the boards and nails off. From your measurements, make a pattern with your pallet wood boards on the ground. Rearrange until you like the “look”. Using a nail gun, hammer, etc begin putting your design wall up. Please be sure and measure, measure, measure! You may paint the pallet pieces before or after you begin the project. You may need to adjust a little in case you didn’t measure correctly. It’s okay!
If you like the look of straight lines in your decor, then I would dissuade you from attempting this project. It will be next to impossible to get the pallet pieces to line up straight, either horizontally, or vertically.
Also if the wall that you are trying to attach the pieces to is not level, it will cause problems with trying to position and attach the pieces in place.