I do not know if you have rocks in the bottom of your pond or not, but if you do it could make installing a bottom drain a lot easier. What I have done before in this situation is to cut a hole half the size of the pipe in the liner at the side of the pond below the water line. Then shove the pipe through the hole and the liner will pucker out along the pipe. Next I used sealant, a large radiator clamp on it, and folded the extra liner back over the clamp to cover it. After that it was simple to lay out the pipe along the bottom and cover it wirh rocks. If you have many leaves that fall into your pond 2" pipe may be a poor choice. I would recomend at least 3" in that situation. I have also used this method on bare bottom ponds, but it takes a while for algae and biofilm to cover the pipe to make it less visible. Have one pond that has been running 10 years now with no leaks. Also make sure that you have the earth packed tightly around the outside of the liner to prevent shifting through the years and pulling at the seal.