This is an example image of the primitive mirror and how to use it.






Here is where you duplicate a body shape of your choosing and bring it to the other side of the mirror. Then flip the pose in the opposite direction.

Next make a duplicate layer make sure to delete the extra body shape on the copy layer. Then go back to the beginning of the first layer delete the first body shape and then the mirror primitive.
Once you do merge the first layer down an empty layer so you can easily erase certain parts you no longer need example is down below.




