Follow the child's lead. The Floortime approach starts with getting the child comfortable enough to like being with the adult. Start out by sitting in the room and talking to the child. Once the child warms up to that, then get closer. Don't go too fast, since this is all about building trust and relationships. Win the child over first.
Join in the play. Once the child is comfortable with the adult, it's possible to play together. At first, just play beside the child. Don't take toys or take charge of an activity. When the child moves on to a different toy, simply follow his lead. Don't try to interfere or give direction, simply do what he is doing.
Be patient. Gaining the trust of an autistic child is not easy. Many times, autistic children prefer playing alone, so it may also take a while for her to accept the presence of an adult, let alone want to play with one. Don't give up. Even though it may take time, the child will progress and accept the adult.
Introduce skills through play. Once the child and adult play together comfortably, skill introduction through play is possible. For instance, label each toy throughout play to increase vocabulary. Show the child new things to do with a toy, such as making cars race instead of lining them up. If the child becomes upset, back off. The point of Floortime is to teach the child in a way that they enjoy. Teach skills that are developmentally appropriate to the child.
Stay positive. Even if the child doesn't repeat the words, doesn't play with toys appropriately, or doesn't even seem to be learning, don't give up. Many times, autistic children have breakthroughs, where suddenly they have an explosion of skills. Of course, this comes only after watching and learning for a long time, then it clicks. Just have fun and play, and the rest will fall into place.