You set up the board. You're pumped and raring to go. You may have even taken a break to grab a cheese sandwich.
Every piece has different rules about how it can move.
But first, keep in mind that only one piece can ever be in a square at a time. If the piece in the square is yours, you have to move it out of the way before you can occupy it. Moe's already in enough pain. Instead, be a classy chess player and say that you captured his piece. You then put your piece in the box on the board, take his piece off the board, and trimphantly place it next to you.