Step 1: is to identify the type of leakage that is occurring and its location. In some cases this may be obvious and in others difficult to locate.
Once you have identified the location of the leak there are a few options to repair them. The following link will take you to a publication with helpful information about repairing techniques. http://www2.ca.uky.edu/wkrec/LeakyPonds.pdf
These methods can be applied to specific areas (at lower costs) if the leaking area can be identified, or they can be applied to the entire pond bottom. In all cases, the affected area will need to be exposed and allowed to dry by draining the pond. While the pond is drained, all vegetation should be removed along with stumps and large rocks. Obvious voids should also be filled with soil.