I have posted a collection of helpful videos to get started with using the Taranis Radio, but its more of a general use case, rather than being specific to Pixhawk.
But my only suggestion would be to play around with the settings and before trying anything on pixhawk just make sure you have 3 channels working (2 of them via one o the sliders/ rotary knobs to control pitch and yaw) and then one 3 pos switch for the modes via the mixer screen. So you will used CH1-4 for roll, pitch, yaw, throttle, CH5 will be for flight modes and then CH6-8 will be for gimbal control. Once you have verified they are working via the monitor screen on the taranis you can then proceed to set it up via mission planner.
Or here is a pixhawk specific video which might help you get familar with how the mixer setting work on your Taranis.
Although I personally only ever use manual pitch control and set yaw to follow mode, and dont manually change modes on the gimbal while its in the air. Reason for no manual gimbal yaw control is that quadcopers can fly in any direction so I control yaw by the IRIS itself, and then the gimbal just smooths out the yaw motion. But I guess its always nicer to have all the options available.
I use the defualt flysky radio with my IRIS so I cant realy help you there.