Calculating a series resistor is pretty straight forward. But first off all do they work correctly when the bike is turned on? If they don't you calculate the following:
Find out the Watt (W or Power) of your old turn signals and the Watt of your new turn signals. Apply this formula ((Watt_old*2-Watt_led*2)/2).
Then find 2 power resistors with the closest Watt number or 4 power resistors with Watt/2.

 

This is the right idea, but not correct.

P_oem = OEM Turn Signal Power, in Watts
P_led = LED Turn Signal power, in Watts
V = voltage of the bike (use 13.5 Volts)
R_compensator = Resistance of compensator resistor, in Ohms. This is what you will need to buy and wire in parallel with the new Turn Signal
P_compensator = Power dissipated in compensator resistor, in Watts. The resistor you buy should have a "Watt" rating larger than this value

R_compensator = V*V/(P_oem - P_new)
P_compensator = V*V/R_compensator

Example:

My 2009 Hypermotard 1100 runs at 12V, or 13.5 with the engine running. Original turn signals are labeled 3.1Watts at 13.5V. Rizoma Sguardo LED turn signals are rated 1.4W at 13.5V. To get the blinker to flash at the right rate, I need to wire a (13.5*13.5/(3.1-1.4)) = 107 Ohm resistor with a power rating of more than 13.5*13.5/107 = 1.7 Watts, in parallel with the turn signal. It is a good idea to use at least a 5W or 7W resistor for turn signals.