1.  Selection of the Parents.  Choosing the right parents for your baby koi need not be difficult, but it can sometimes be tricky too.  First things first - you need to know which of your koi are female and which are male.  Female koi are visibly rounder than male koi, especially those that are ready to lay eggs.  Males are slimmer in appearance, and may develop roughness on their gill plates when ready to spawn. 

Choose mature koi only for breeding, i.e., they should at least be two years old (younger koi will produce weak offsprings).  Experts say that the optimum breeding age is 2-4 years old.  Choose only healthy koi with no deformities whatsoever.  Both parents should exhibit excellent body conformation and high-quality colors and markings.

Select the parents based on what baby koi you're after.  For example, if you want Kohaku fry, then you need both parents to be Kohaku.  Some varieties don't result in nice koi when bred together, so be sure to do a little research on koi variety pairing before doing it (unless you're after the excitement of uncertainty).

Some breeders use two males for a single female during breeding to maximize the yield of the propagation.  The breeding act of the koi is very physical and can harm the participants (especially the female if the males are very aggressive), so this ratio of 2:1 must not be exceeded.   One advantage of using just a single male is the higher predictability of what the offspring will look like. 

2.  Preparation of the Spawning Environment.  Once the prospective parents have been identified,  they need to be taken out of the main pond and isolated in separate and smaller ponds where they can be conditioned for spawning. Males are separated from females to prevent indiscriminate spawning. Many hobbyists start this isolation at least 1 month before the anticipated spawning date.

Eventually the female becomes rounder and noticeably bloated with eggs.  Now with a heavy but soft abdomen, she is presumed to be ready to lay eggs and is very carefully moved to the spawning pond.  This pregnant koi must always be supported by water during the move, even while inside a net.   At this point the male is also assumed to be ready to participate in the reproduction as well, and is moved into the spawning pond a few hours after the female has already been acclimatized to it.  Many breeders introduce the male in the evening, since spawning usually happens in the wee hours of the morning.

The spawning pond shouldn't be big - usually with an  area of just 6 to 12 sq. meters. It should be thoroughly cleaned and filled with un-chlorinated water to a depth of about 50 centimeters. The spawning pond must have a generous amount of spawning material to encourage the female to lay her eggs on them.  Many modern koi hobbyists prefer synthetic spawning ropes (see Figure 1) as spawning material because these are free of parasites, do not easily get damaged, and allow easy handling of the eggs, unlike spawning media of the natural kind.  The easy-to-handle feature is important if you plan to move the eggs away from the parents right after spawning is completed.  Many old-school breeders still prefer natural spawning material though.

Figure 1.  A curled spawning rope (left) and a close-up of the spawning rope bristles (right);  Photo sources:  koilogic.co.uk; keepkoi.com

The spawning pond must also be sufficiently aerated at all times.  Strong but silent air pumps must be used.  Unlike the main pond, aeration of a spawning pond must not result in water turbulence, since water tranquility is needed during spawning.  As such, the aeration system of the spawning pond must be designed well to meet the aeration requirements without disturbing the water.