How to write a custom Gazebo plugin for ROS ?

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I'm not sure how up-to-date it is (especially with the recent releases of newer versions in the 7.x and 8.x series of Gazebo), but gazebosim.org/Tutorial: Ros Plugins would seem to be the "hello world" of writing Gazebo plugins that integrate with ROS (with gazebosim.org/Tutorial: Using Gazebo plugins with ROS showing how to configure your urdf/sdf to include the plugins in a model/sim).

I would also recommend to take a look at gazebo_ros_pkgs (source: ros-simulation/gazebo_ros_pkgs) as that package provides most of the standard Gazebo-ROS plugins. Personally I find reading other peoples sources very informative.