To get a golden watering can, you need to maintain a perfect town in Animal Crossing: Wild World for 15 days. Having a perfect town means keeping it clean and having a good number of trees and flowers distributed evenly throughout the town. When you have a perfect town, you will randomly discover Jacob’s Ladder flowers in your town. Those types of flowers will only appear if your town is perfect.
Your town is divided up into 16 acres. Each acre is 16×16. What this means is that if you dig 16 holes in a line from the top of the town and 16 holes from the left of the town, you have measured one acre. You can also measure out acres with designs on the ground, or flowers, or whatever. The maximum number of holes you can dig (or flowers you can plant, etc) in an acre is 256.
To have a perfect town, at least 8 of the 16 acres must be considered perfect. You don’t have to have any trees an acre for it to be a perfect acre, although you will have to have a bunch of flowers in it to make up for the lack of trees. If you do not want to have to maintain a bunch of flowers all the time, put 12, 13, 14, or 15 trees into an acre, and you will only need to have three flowers in that acre for it to be considered perfect. For each number of trees fewer than 12 or more than 15, add two more flowers to make it perfect. So having 11 trees means you need 5 flowers, and having 10 trees means you need 7 flowers, and so on.