Where to fit the fuel saving device?

I bought one of the fuel-saving device. but I do not know where we put. (where fuel hose ?)

transport: 2003 model 1.4 automatic transmission 55.1kw

can show the picture goes a long way.

Thank you in advance.

but I do not know where we put.

In the bin because they don't work (otherwise Citroen would have fitted it at the factory and advertised a car with better economy)
https://en.wikipedia.org/wiki/Fuel_saving_device

(where fuel hose ?)

The fuel hose supply for the petrol engine runs from the rear of the car (fuel tank, fuel pump and filter are under the rear seat, below the big black plastic cap), under the bodywork and into the engine bay fuel rail. The fuel rail is attached to the injectors, find the injectors and you have found the fuel rail.

This is a picture of the fuel rail, off the car. The arrow identifies the fuel input, there will be a pipe from the fuel tank attached to it. this is the fuel rail with the injectors attached, as you will see it in the engine bay. Look in the area of the white arrow to find the fuel rail, then you can find the fuel rail feed pipe (that comes from the fuel tank)
If you would like to save fuel, keeping the tyres at the correct pressure, keeping rapid acceleration to a minimum, reading the road ahead to reduce the need for heavy braking and then accelerating can all help. Other things that can help are to remove unnecessary weight like a tool box from the boot, remove roof racks or roof boxes when not in use, keep up with the maintenance, like air filter and plugs, keep windows closed when travelling at speed and (use the A/C (topic)).

C3s might only be little cars but they have a sophisticated engine management system which is there to get the optimum performance versus economy from the fuel used. Mess around with the systems at your own risk unless you know exactly what you're doing. Petrol is very dangerous when not properly contained, interfering with fuel lines which are under pressure can result in a significant chance of fire and loss of your car. Ask yourself, are you sure of the quality of the device you propose to fit and as C3driver52 says, if they really work, why didn't Citroen think to fit one?