The first requirement is to have money. The currency in this game is Florins. You will no doubt be getting a lot of these in the game, but if you want to get it right away just steal money from the various people walking around by pressing near them.
Wells can be found throughout Rome. There is one just about in the middle of the map, on the far East side of Centro District. It's right in front of the gray building on the map, by the bank and the blacksmith. Wells will have a rope and bucket, with an outer octagon shape. There is wood around it as well, so if you find a hole and it doesn't look like this, it's not a well. To throw the money, hold and select the money bag icon. Now face the well and press and the achievement will unlock.