Think of the R/2R ladder as a simple digital to analog converter. You connect any number of outs from your gates, shift registers, and counters to the R/2R and that output will provide a voltage to send to your VCO. There is an order to the R/2R in which you have the Most Significant Bit all the way down to the Least Significant Bit. Playing with this order alters the output quite a bit.
Need a VCO? Take the easy way like I did and have something quite versatile: buy a couple of cd4046 ics. The drawings below can be done on separate panels or on the same one. You could get interesting results just taking the R/2R output straight to your mixer but running those voltages through the vco offers another variation on the sound.
Here's an audio sample