This Instructable is for the Cheerson CX-20 with a different APM board than most other people have. These steps will explain what I mean. There is very little instruction for setting up telemetry with this board, but I managed to figure out how to get it working properly.
Step 1: Telemetry Unit
This is the Telemetry unit that I am using. It is a 915Mhz model and it comes with the necessary components to install the telemetry.
Here is a link on amazon for this exact unit:
Step 2: Opening Your Quad
The first thing you need to do is take the top part of your quad off. They supply the tools.
After that you need to locate the box that is in the middle of the craft and open it. There will be a zip tie around yours, you just need to cut it and you can put another one on when your finished if you wish. Once you open the little box there is a piece of foam. That needs to go back on there in a similar fashion it came off.
Step 3: Accessing the Port
After you open it up, take it out carefully and flip it so you can see the underside. There should be a port already soldered onto the board and labeled UART. If this is not there, this Instructables is not for you.
Find the cable that came in your telemetry box that fits the port and plug it in.
Step 4: Clipping Some Wires
On the end of the cable that plugs into the port on your APM board, clip the Green and Yellow wires. I would clip them pretty short, you won't be using them on that end of the cable. On the other end (The end that connects to the remote telemetry unit) Leave the wires long and strip the ends.
On the board the Ground and +5v are switched, so you will need to cut them and solder them to the opposite wire. So Red wire will now go the the black one, and the black one will go to the red one. Don't ask me why this is I have no clue.
Step 5: Soldering the Tx and Rx Wires
Take 2 extra wires and solder them to the location on the bottom of the first board. Wheres the first board you say. Well just like and arduino, the boards come apart. So carefully pull the boards apart and take the top board and flip it over. It should look like the picture. Those 2 extra wires need to be soldered to those 2 points on the board. These are Rx and Tx pins. Now it doesnt matter which wire goes where because if you mix them up the device simply won't work and you will have to switch them around.
Step 6: Connect the Rx and Tx
Now you need to solder those wires from the bottom of the board to the cable. You need to use the green and yellow cables that attach to your telemetry device, not the ones connected to the UART port that we clipped earlier.
In my case the White wire from the previous step goes to the green wire and the Black wire goes the yellow wire.
It may be different for you, so to check you need to go to your computer and see if both the ground telemetry (USB end) and the remote telemetry can connect. There should be a solid green light and a blinking red one. If the red one on the remote side is dim, the connections are wrong and you need to switch them.
Step 7: Finishing Up
Once you have identified the correct wiring, then your all set to put it some heat shrink tubing on for neatness and put it all back together. Your cheerson should now talk to mission planner