How do I transfer from Gare du Nord to Gare de Lyon

The RER train platforms in Gare du Nord are both below surface level and near the same area.  The moment you step off the RER B train and leave RER B platform, there will be signs for RER D trains which will travel to Gare de Lyon.  The RER D train direction of Melun, Malesherbes, or Corbeil / Essonnes will all work for Gare du Nord to Gare de Lyon.
 
Gare de Lyon will be your 2nd stop, just after Chatelet Les Halles.