Ask anyone who has built a pond and the first thing they say is it's too small so build big. Deep water for koi. You first lay out where you want the water, then dig the hole. Take into consideration that you may want to put some potted plants down in the water around the edges so make a ledge for them. Next you cover the hole over with a pad that will keep the roots/rocks from punching holes in the liner. Add the liner, then edge everything with stones to hold the liner down. Don't get carried away cutting the liner until you have the stones in place. Measure twice, cut once..... You will need a filter system and pump which can be placed in the deep end of the pond. Talk over with an electrician where/how to run power for everything. To get a feel for the cost, check out liner/filter/pump etc prices and what your time is worth.
