Normally its fitted between the intercooler/ chargecooler and inlet manifold, the difficulty is distributing the waterspray evenly to each cylinder, (depending on the shape of the inlet manifold, the waterspray may have a different flow to air and each cylinder may recieve different quantities of water - the old fashiond way to check is a plug chop) sometimes a nozzle is used per cylinder, close to the inlet ports on the head, to inject the same amount to each cylinder

I have not actually fitted a water injection kit to my car but researched/ considered a water/ meth kit, decided against it (complicated, can be difficult to set up, different setup with/ without, water/ meth tank adds weight/ can run out - may be more practical on a pure drag car) and fitted an external waterspray to my intercooler to go with the other temp control stategies (heatwrapped exhaust/ manifold/ vented bonnet/ insulated cold air intake/ electric waterpump)