Hi Daan:
Look at this schematic:
http://www.casperelectronics.com/images/finishedpieces/speak-n-spell/Speak-n-Xbending/LFO%20schematic2.jpg
The point labeled "C". This is the pitch base point. COnnect this to the center leg of a 100k potentiometer. Next connect the right leg of the pot to various points on the board. You will eventually find a point on the board that makes the pitch higher when you turn the knob. NEXT connect the left leg of the pot to various points on the board until you find a point that makes the pitch go down. Now you will be able to make the pitch go up and down by turning the knob.