Drain your pond. Depending on the type of pond you have on your property, you can either do this via a pre-installed plug or with a water pump. You could be old-fashioned and use a bucket but that would be quite time intensive. Once drained, allow it to dry overnight.
Apply a coat of pond sealer using either a paintbrush or a paint roller, depending on the size of the pond. Coat the entire pond to ensure that the sealer maintains its integrity for the longest period of time.
Even the sealant with a squeegee to make sure that all areas of the pond interior have a smooth, even coating.
Allow the pond sealer to sit overnight. This is so that the sealer has the chance to seep into the leaking areas and harden.
Refill the pond with water and check for leaks. If you've properly coated the pond with the sealer, you will not find a stray drop.