Batter boards are horizontal pieces of wood used to help square and level the post footings. There are various HOWTO sites and videos for how to construct them. The method I chose was to set 3 stakes at an approximate right angle, and then attach scrap 2×4 corner pieces.
I used 1×2 stakes like these, which are helpful for marking things. But, they proved too be really flimsy and hard to attach the corner pieces after they are set into the ground. Several of the stakes split, and I worked around this problem by drilling pilot holes. Still, using a rubber mallet to level the batter boards caused some of the stakes to crack.
Next time, I’d make batter boards before setting them in the ground. trying to fasten and level a crosspiece to two flimsy stakes after the stakes have been set in soft ground is a tedious exercise. I’d also step up to 2×2 stakes instead of 1×2, to improve stability and reduce risk of the stakes splitting.