When thinking about installing a koi pond, I suggest that you research as much as possible before you begin and definitely look at pictures of what others have had built for ideas. Always use a licensed contractor who has liability insurance and has good reliable references.
Here are a couple of examples of what we at Garcia Rock And Water Design have created in Santa Barbara, Ca.:

Waterfalls for Koi pond aeration.

waterfall into pond

Great Koi pond filtration for crystal clear water

Koi in a dark bottomed pond
Koi in a dark bottomed pond

Pond Construction Santa Barbara

Lots of room for koi fish
Lots of room for koi fish
Very nice garden pond
Very nice garden pond

Santa Barbara Koi Pond Builder

Front Entryway Garden Pond - designed and built by us
Front Entryway Garden Pond – designed and built by us

Waterfalls in Hope Ranch, Santa Barbara

Koi fish are easy to see in a dark bottomed pond
Koi fish are easy to see in a dark bottomed pond
Gravel filtration helps to keep the water clear
Gravel filtration helps to keep the water clear