For random encounters, right-click the map you want the encounters to take place on and click 'Map Properties...'. Under 'Encounters' (right hand side), click the area under 'Troop' and select the battles you want to take place (underneath that area, you can change the Steps Average option to change how often battles occur).
For specific encounters, make an event and use the 'Battle Processing' event (Page 3) to select which battle you want to occur.