The easiest way to create straight vertical corners when building with earthbags is by using corner guides in each corner. These are typically built with two 2x4s nailed together to form a right angle. Secure each corner guide with a stake at the bottom and two angled braces, which are also staked. (Note how we use poles instead of milled lumber to save money.) Plumb each corner guide with a level as braces are staked and nailed in place. Be sure to routinely check all the corner guides throughout the building process to ensure they are not bumped out of plumb.
My YouTube video demonstrates the process.
Video text: It’s good to use corner guides made with wood to help keep corners plumb and straight. Check for plumb in both directions. We have strong braces that hold it in position. And we have stakes at the bottom. The corner guide is simply made with two 2x4s nailed together. And it goes right in the corner.
My YouTube channel shows each step of earthbag building in short video clips.