The thermostatic valve goes on the flow pipework, i.e. the inlet to the radiator. The best way I can think of to balance the system would be to set all the thermostatic valves to their highest setting and open all the lock-shield valves on the return pipework. Then go to each radiator and close the return valve untill the flow and return temperatures are roughly the same on all the radiators. (test this by holding the pipework attached to the radiator) Eventually you will get a fairly equall flow through each rad.