Assemble the parts of the circuit in red and green. Do the red part first. Do not connect power until after you have finished putting both the green and red parts together. The part of the circuit in red is to leave no floating input pins on the CD4049 chip, as otherwise it will not work correctly. When you poweer the chip, power with +15 on the first pin, and ground the 8th pin. The green part of the circuit is the first amplification stage of the distortion circuit. The switch allows you to choose between a high and low level of gain in this stage. This IC has a very non-linear gain so the more amplification you choose, the more distortion you will get.
When this circuit is complete, supply a sin wave input of 1Vpp at 500 Hz. If you've done this right, it should like the image below when you hook it to a scope. For a low R5 value on the rhythm lead, you will only see a slight distortion from the original wave.
Tested the lead switch. While on lead, you will get non-symmetrical waves with sharper peaks. The gain should always be greater than 1. In the lead position the gain should be 6x. It should look something like the image below.
Now that the green and red portions of the circuit are confirmed to work, build the blue section along with the output potentiometer and attach it as described in the circuit diagram to the green part of the circuit. Make sure power is turned off while you are building this. The purpose of this part of the circuit is to act as a secondary amplification stage. If the first stage does not add enough distortion the secondary stage guarantees a tube fuzz sound. The potentiometer at the output acts as a volume dial.
Now test the output of the blue portion of the circuit. For low R5 on the rhythm switch, the output should look like the following image given a 1 vpp 500 Hz sin wave input.
Now test the lead configuration. The output should look like the following given a 1 vpp sine wave input at 500 Hz.
Congrats! Your circuit is now finished and you can connect any sort of line level input and you will make it sound like rock and roll.