As Augmented Reality (AR) has become more popular in museum mediation and storytelling on Cultural Heritage (CH) sites, the need for easy–to–use tools to develop applications has become evident. This paper provides a background to the how-to session introducing the Sitsim AR Editor—an editor for putting together applications in the form of situated simulations (indirect augmented reality). The Sitsim AR Editor is an add on to the Unity 3-D engine, a game developer platform often used for non–game purposes, and is based on ten years of experience with app development for museums and CH sites, including Forum Romanum in Rome, the Acropolis in Athens, Ancient Phalasarna on Crete, Viking settlements and burial sites in Norway, the baroque Old Town of Narva in Estonia, and the D-Day landings site on Omaha Beach in Normandy. The how–to session is organized as follows: 1. Introduction to the Sitsim AR platform as a storytelling device for museums and CH sites. 2. Introduction to the Sitsim AR Editor, including how to import, position, scale and orient 3-D terrains and objects, create content links and build an application. The Sitsim AR Editor is currently MacOS–based.