There are many ways to accomplish creating a chalkboard label on a bucket. One of the easiest ways is to use chalkboard contact paper.
Simply cut the chalkboard contact paper into the size and shape you need, peel off the back and apply it to your bucket. Grab some chalk and label away.
Another option is chalkboard paint. When going with this option you will want to sand the surface that you are going to apply the paint to so that it will stay on your bucket. You can either purchase your own chalkboard paint, or you can make your own, (tutorial at Attempting Aloha). You can go all out and paint your galvanized pails entirely with the paint, or you can tape off the area/shape that you desire. Once done, allow to dry, grab some chalk, and create your labels to use your buckets for storage.