Drain your pool. Take photos of your pool from various points in your yard, looking toward your house, and from various vantages inside the house. Note the lines of sight. Note patterns of shade throughout the day as well. Consider whether to create a shade structure for part of the pond.
Give the entire pool a muriatic acid wash, unless you have a pool liner. See Step 3 for what to do with the liner. Wear rubber gloves, eye protection and a NIOSH-approved respirator while applying the muriatic acid to the sides and bottom of the pool. Fill the pool with water and run the pump for several weeks. Drain the pool and repeat three times.
Remove the pool liner, if there is one. Examine the ground under the liner. If there is any evidence of past liner leaks, dig out 1 to 2 feet of soil and have it carried away. Replace with a 6-inch layer of large gravel, tamped three times with a reversible pressure plate compactor. Cover with a 6-inch layer of triple-compacted sand, followed by a layer of smaller gravel and a final 12-inch layer of sand.
Decide the shape, size and depth of your pond. You can keep the pool size and shape as they are if you choose. Straight sides, rectangular or ovoid shape with a sloping bottom are fine as is, although you may find it more aesthetically pleasing to modify the existing pool to give it a more natural look. Use sandbags to create a new inside shape. Backfill the corners of the pool with layers of gravel, sand and soil to create flower beds, or gravel alone to create biological filters. If you are going to make biological filters, run any electrical conduits, plumbing lines, air intakes and water outlets, then line the corner and fill with gravel. Do not use sand in your biological filters, as it can enter your pumps.
Use a backhoe to expose the walls of your pool, if you need to add any drains, pump outlets or water intakes. Use a masonry saw to cut through the walls of your pool, install the necessary drains, water lines and pumps, and carefully repair the breaches.
Pull your pond liner into place and shape it to the walls as desired. Eliminate as many wrinkles as possible. Wrinkles collect debris, which can decompose and affect the pH and oxygenation of your pool. Seed your pond with water and gravel from an existing biologically balanced pond, or add two or three sacrificial feeder fish to get things started. Fill your pond and run it for a week or so while your bacteria colonies get established.
Add fish one or two at a time when testing reveals no ammonia and no nitrates, waiting a week in between for the filters to adjust and the beneficial bacteria in your pond to respond to the increased ammonia load.