Which is TX which is RX
Channel 2 goes to the tx uart on the flight controller. Channel 1 goes to the rx uart on the flight controller. The TX/RX uarts that you wire to on the flight controller must be in the same pair of UARTS. 5v in requires 5 volts from the flight controller. Wire ground to ground because, ground is ground is ground.
In the example below we are using UART1.
Not all uarts are equal
There are some times that a tx or rx uart might be dedicated to a specific type of inversion on some types of motherboards to be compatible with different receivers. On an F4 board, you do not want to use these UARTS.
If you are not sure, check your flight controller documentation if possible.
An example of this in the documentation for the F405-STD.
If this was an F3 or F7 board, this would not be and issue and you could wire to it. Because F3 and F7 boards can handle inversion on the fly.