Once you find your pallets (lots of people just give them or away, or sell them for $1 each on craigslist), measure to see how long/ wide a deck you will want. Also, since pallets vary so much, you will need to see how sturdy they are, and if the gaps between each piece of wood are too big to have furniture moving back and forth without catching (that was the case with the pallets we had). As you can see from the picture below, the pallets we had only had 1 2x4 in the middle (making it very unsturdy) and they had pretty big gaps in between each piece of wood.
Since it was a 2x4 in the middle, that is what we bought to reinforce each side. We cut them to length and stood each pallet up on its end and pounded them down with a hammer (they are not attached at all as it was very snug fitting them). This took care of the sturdiness problem - we no longer had any worries someone would jump up on the deck and break a piece of wood with their weight! Here you can see there are now 3 2x4s in the pallet.
Then, to take care of the gaps on top, I went to Lowe's and found rounded 1xs that fit in between each gap on top. (Tip: Measure the smallest gap between your pieces of wood so you know it will fit each one). We cut each one of those to size, pre-drilled holes in them (important so they don't crack!) and attached those with screws.
Then, you sand, stain, paint or whatever you want.
Our deck was a total of 12 individual pallets put together. We had pavers we leveled up (with all of the mud of course!) and put a corner on each one. Meaning the the pavers for the center of the deck held 4 different corners of 4 different pallets; the end pieces utilizing an entire paver itself.