Custom Sensor Integration and Monitoring

Hi
I have been searching day for a straight forward solution or at least a clear starting point for my project.
I want to sample some custom sensors… My sensors are being sampled and processed via a separate Arduino board. Then I can send the sensor data using ADC Channels, Serial, I2C , etc. I want to sent this values to Pixhawk. Create a mavlink message for them and then I can monitor the values in Qgroundcontrol analyze plot and data monitoring as MY_SENSOR_1…
I also want to include them in the log file stored in SD card…
Can you please help me where to start and what resources are out there?