I have to suggest that you follow the guidelines in the document Greg mentions. Anything else, you're sort of on your own.
If you get the 0-100-0 effect with a mono cable, this means that you are using a volume pedal rather than a pure expression pedal. You can fix this by cutting off the wire going from the ring connection on the jack to the control. Or, you could replace the mono jack plug with a stereo one, and leave the ring unconnected.
to confirm I have followed the guidance exactly as the document sets out but it does result in the two alternative scenarios I described above. I am going to try replacing the mono Jack with a stereo one as my final attempt. I do have soldering iron but I have never attempted making my own leads before. Apologies for being a noob but it would be great if you could confirm the following connections.
I will snip off and strip back the mono Jack connected into the boss pedal. After stripping back the cable to expose core wires and braid, I solder the core cable to the central pice which will connect to the tip of a new TRS Jack, and the braided outer to the outer element leaving the ring (middle connection) not connected?