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The main concern is the button/axis mapping in the subscriber. Each robot interprets joy messages differently. You need to know what fields you have to emulate. For example, the Xbox 360 controller works as such:

#define XBOX_AXIS_LHORZ     0
#define XBOX_AXIS_LVERT     1
#define XBOX_AXIS_LTRIG     2
#define XBOX_AXIS_RHORZ     3
#define XBOX_AXIS_RVERT     4
#define XBOX_AXIS_RTRIG     5

If you wanted to publish to the left stick vertical axis (up/down) to tell the robot to go forward, you could do the following:

sensor_msgs::Joy joy_msg;
joy_msg.axes.push_back(0.0); // Left Horizontal
joy_msg.axes.push_back(1.0); // Left Vertical (-1.0 through 1.0)
joy_msg.axes.push_back(0.0); // Left Trigger
joy_msg.axes.push_back(0.0); // Right Horizontal
joy_msg.axes.push_back(0.0); // Right Vertical
joy_msg.axes.push_back(0.0); // Right Trigger

#define XBOX_AXIS_LHORZ     0
#define XBOX_AXIS_LVERT     1
#define XBOX_AXIS_LTRIG     2
#define XBOX_AXIS_RHORZ     3
#define XBOX_AXIS_RVERT     4
#define XBOX_AXIS_RTRIG     5

If you wanted to publish to the left stick vertical axis (up/down) to tell the robot to go forward, you could do the ~~following:~~following in the publisher:

sensor_msgs::Joy joy_msg;
joy_msg.axes.push_back(0.0); // Left Horizontal
joy_msg.axes.push_back(1.0); // Left Vertical (-1.0 through 1.0)
joy_msg.axes.push_back(0.0); // Left Trigger
joy_msg.axes.push_back(0.0); // Right Horizontal
joy_msg.axes.push_back(0.0); // Right Vertical
joy_msg.axes.push_back(0.0); // Right Trigger

publisher.publish(joy_msg);

The .push_back() method appends a new value to the "axes" vector. If you don't add to the vector, it will be empty. If it's empty when you try to access it with axes[i], you'll get a segmentation fault. The above code will generate a new message for you to publish to your robot.