How do I join two pieces of plywood together?

Builder's choice, on that one. Depending on the length of the mold and how many external braces are required, I like to stick with whatever will make disassembly easiest without having to pry against your fresh concrete - a trim nailer is usually sufficient, but plain old all-purpose construction screws also work fine. Remember, you wont be leaving the mold on there until it's fully cured - just long enough for the concrete to set up to the point that it won't collapse under its own weight; usually only one to three hours. Remember to coat the inside of your form with fresh motor oil (paintbrush works) for easy release.