How do you make a weighted base for sign post?

Depending on how nice you would like it to look, you can pour cement into a bucket of proportionate size with the post in the middle of it, using string or another device (several layers of aluminum foil?) to keep it in the center while it sets up. If you are sure that the cement is set and uniform, you can break off the bucket and have just the cement there, or if it looks good enough, you can leave the bucket on.