Clear the excavation site of any loose debris. Place markers, such as stakes, denoting the corners of your site.
Attach a loader bucket with teeth onto the digging boom. This kind of attachment is best for straight digging.
Lower the boom toward to the ground while adjusting the bucket, so it enters at a 90-degree angle. As you dig deeper, keep the bucket as close to 90 degrees as possible.
Remove the bucket, and attach a planer blade to the excavator.
Lower the planer blade into the hole, and use it to smooth out the straight walls and make them more even.