Grab the schematic and follow along.
A white noise source is built from Q1 and U1. Select the noisiest transistor you can find for Q1. Adjust R6 to get a good signal level. The white noise is then filtered by U2a to get pink noise. The pink noise is filtered by U2b to get red noise. The red noise is filtered by U3b to get infra-red noise. One of the four colours of noise are selected with switch SW1. The selected noise goes to three destinations: to the Audio In jack via U3a, to the Sample & Hold signal input, and to the Direct and Wheel modulation buses via pot VR1 and switch SW2.
Note that this noise source replaces the Pro-One's original noise source. To do this, you need to cut a trace on the Pro-One motherboard. Look for U107 pin 14 where it connects to R1116, that junction is the output from the Pro-One's white noise source. Cut the trace just prior to where it reaches the switching lug of the Audio In jack. Now connect the new noise source to the switching lug. The external Audio In jack will still function normally. You can still use the new noise source for the Sample & Hold and modulation buses even when using an external audio signal.
The two connections to the Direct and Wheel modulation buses are best done by attaching R20 and R21 right to the Pro-One motherboard near the summing nodes of U101.