It all depends, you'll have to provide more info and detail.
There are many types of flush kits and mechanisms, from simple to complex. I have an inline system between the drives and the raw water intake similar to this one here. . It requires hose pressure first to close off a spring loaded valve, then turn on the engine to run for up to 15 minutes.
There's another type like this one here which does not require the engine to be started. Then there's the real simple do-it-yourself type with a valve and manual lever.
Try to elaborate on the set up, somebody will be able to provide some advice.