Building the field table would require some grown-up help! FLL has a nice set of instructions for you to build the field, but you will need an adult to help you purchase plywood and any equipment you need, along with helping you cut the wood, etc. We wouldn't recommend you do this on your own! Below are FLL's instructions on how to build the field table (pdf & a video).
The user-friendly video below shows how you and an adult can build the field table. Tip: Adding wheels to one side would help you make the table portable (you can roll it one side, and move it around more easily).