attach them instead of skin sensor you mean in A0 analog pin ? because that where the sensor goes also where i will i attach sensor on the board after it?
No, the Grove sensor board has a terminal where the actual sensor is connected. You have to attach the known resistor values there. Keep in mind that these resistors are key for the accuracy you will get. So if you attach a resistor with 5% accuracy (the default) the measured value will have an accuracy of +/- 10%. So to get useful values you should use resistors with 1% or better.
I found the page where you got the equation from. There they describe a very simple calibration method. If you do that you will get values in Ohm with the mentioned formula but you have no clue about the accuracy.