As I'm the person who drew that diagram
6 years ago, it's a lot easier to understand with the instructions that went with it.
Full instructions to fit it.
The Rate Pot wants to be a potentiometer with a 1/4" shaft that can be screwed through the case so you can adjust it.
The trim pot can be any small potentiometer, size doesn't matter although I use a subminiature one designed for circuit board mounting with the centre pin at the opposite side of the pot, such as Maplins part number UH16S, so it can be soldered directly to the pin on the rate pot.
1. Remove back of transmitter and drill a suitable hole for the rate pot, making sure the back of the transmitter still fits okay. Attach the steering rate pot to the case.
2. Rewire the transmitter following the diagram above, the green lead in the diagram was the wire that comes from the PCB to the middle pin on the steering pot, move it to the rate pot and add all the other wires.
3. Set the steering trim below the stick to the centre, switch on the transmitter and the car. With the stick at centre, turn the Rate Pot fully clockwise and anti-clockwise. The steering should not move, if it does readjust the Trim Pot until no movement is seen.
4. With the Rate Pot fully anti-clockwise, full movement of the stick should produce little movement. With the Rate Pot turned fully clockwise full movement of the stick should produce maximum steering. If the reverse happens swap wire on 1 & 2 on Rate Pot.
5. Tidy up all the wires in the transmitter ensuring none foul the stick mechanism and there are no possibility of short circuits.
Now replace back of transmitter.