You then need to cut the power wire going from your battery to your pump/flow controller, strip off the ends and put one end to C and one end to NO. This is a break in the wire, so that when you press the remote switch it turns on and off.
Personally, I would use the C and NC, not NO.
This would mean that the remote is switching OFF (rather then switching ON) the water flow. The overall effect will be the same, ie you can switch the flow on and off remotely.
Why?
Because if the remote is switching ON the water, and gets damaged, lost, malfunctions in any way then you will not be able to switch on the water flow, and therefore you will not be able to work.
If the remote is switching OFF the water, then if theres a problem with the remote function, the worst that can happen is that your system will function as if it had no remote. IE you will have to swich the water on and off at the flow controller, the same as you do now. It will not stop you working.
Just my opinion.