The most important step is the liner... overlap it into the pond... best bet is to join it to the pond using roofing cement... thats the only compound I found to work effectively.
As for everything else... I used cinder block for a base, then dirt, then the liner. The hardest part for me was the falls. I rebuilt them three times before I changed the design. Don't be concerned if you feel you have to rebuild it three or four times before it comes out the way you want. Once you get the final design, then use the foam if the water isn't cooperating.