I prefer heavy duty 2-gallon plastic buckets for filling earthbags, the same kind used for bucketing cement. They’re strong and small enough that everyone can handle them comfortably. In fact, they’re just the right size. Because they’re larger than the metal cans typically used, work progresses more rapidly. Two rounded shovelfuls fill a 2-gallon bucket perfectly. Or you can save your back and pull the material into the bucket as shown.
Each bag (ours are about 18” x 30” when empty) takes four buckets of soil. In some cases only three buckets are used so as to avoid ending a bag above a previous joint. (Always stagger the joints as in masonry construction — “one over two.”)
Leveling: Measuring the contents of each bag and placing them with a bit of care is the best way to go, much better than just random filling. The walls are almost self-leveling if the bags are the all same size. Only an occasional bag will require a little extra tamping to obtain very accurate results.