Because you are not going to operate the motor under a high slip condition, you simply short out the rotor windings. This can be done on the rotor itself and the brushgear removed. No resistors are required. Mark Empson
Marke is right. All you have to do is short the rotor windings. Take out the brush gear and short the rings. The best way to do this is to get two pieces of copper bar and put them 180 degrees apart so that the rotor is balanced.