First about the connection plan: You need a power supply that supports +9V,-9V and 5V. Connect all pins. You have to connect D0..D7 from the adressbus with the Z80 STI. You have to connect D0 with pin 21, D1 with pin 22, ..., D7 with pin 28. That's all.
Now you can plug in the interface in the CPC. Switch the power on. If the CPC doesn't work normal please switch the power off and you have to watch for faults in the connection of your interface :-(. If everything is ok then you have to copy the assembler code and do the next test.
'''Bugs and Confusing things in the GIF schematic:'''
* Confusing: Signals 0..15 on the left side of the schematic refer to address lines A0..A15
* Bug: Z80-STI Pin 3 (TCO) should be wired to Pin 39 (RC), not to Pin 34 (A1).
Second part: How to program the interface. (The assembler code is saved in RS232.ASM)