Process of Buildding map

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Hello Helpers.. I'm new in ROS and worked with topic, services and action and some robotos like wanderbot and teleop bot successfully. But now i stuck with Building maps please share the process of Building maps.. Some people says that we need a launch file for gmapping but i don't know how can i build the launch file and for what it is needed. Please help me i am trying from last 6 days but no solution...

Thank you.