how to create a the launch file xml

edit

i follow the tutorials to 2.3 The launch file .the tutorials just tell us to :"Now let's create a launch file called turtlemimic.launch and paste the following: "but it doesn't tell us how to create a launch file .so ,i really don't know where to past the scripts it provides :

<launch>

  <group ns="turtlesim1">
    <node pkg="turtlesim" name="sim" type="turtlesim_node"/>
  </group>

  <group ns="turtlesim2">
    <node pkg="turtlesim" name="sim" type="turtlesim_node"/>
  </group>

  <node pkg="turtlesim" name="mimic" type="mimic">
    <remap from="input" to="turtlesim1/turtle1"/>
    <remap from="output" to="turtlesim2/turtle1"/>
  </node>

</launch>

so ,could any can tell me how to create a launch file and where to paste the scripts .