This is a guide on how to install 3DR Radio Telemetry to the APM 2.5 board. By default, the installation is easy and does not involve a lot of tinkering. What you would need are an APM 2.5 board, Li-Po battery, land telemetry module and air telemetry module (see figure below).
1. Observe the air telemetry port on the APM board.
2. Connect the air telemetry socket to the APM board.
3. Connect the Li-Po battery to the APM board. Now the LED on the air telemetry module should be blinking.
4. Connect the land telemetry module to the USB port on your computer.
5. Look at the top-right, you should see that the Mission Planner and the APM 2.5 is not yet connected. Take note on which COM port your land telemetry was installed. Mine is COM 4. The baud rate shown besides the COM port should be set to 57600. Once done, click CONNECT.
6. A pop-up (Connecting Mavlink) should come and shows that the parameters are being downloaded from the board to the Mission Planner.
7. After a few seconds, the APM board and Mission Planners should be connected. The CONNECT button turns green. Now you can see the battery percentage, accelerometer level and set the waypoint wirelessly from your laptop to the APM board.
8. Finally if you hate the mess, get some cable-tie to clean up the arrangement on your quadcopter