If you have a choice in your pallet selection, choose them based on what you are building. They come in a variety of thicknesses and widths which may be a factor in your design.
Other things to think about are how rough sawn the wood is, whether you are sanding it and certainly the color as well. So if your project will require multiple pallets, be sure they are the same.
Sometimes the better side of the wood is on the inside. So check there too.
Once you have your pallets on hand, it’s time to start cutting. I prefer to just cut the ends off. It saves time because they are often split and you need a straightening cut anyway.
Then flip it over and cut the other sides, removing the 2x4s on both ends. Now moving on to the removal of the individual slats and finally the nails.
Hammer each nail backwards through the wood, then turn it over. Use scrap wood under your hammer head to protect the wood you are saving from the pallet.
Finally you have a nice little pile of pallet slats that can be up-cycled for your project. It will probably take more pallets than you expect. If you have extra, then you have a jump start on your next project.