Bottom is signed just like below. Start the sign for bottom by taking your non-dominant hand and laying it flat and horizontal in front of your body, as if making a floor with your hand. Hold your dominant hand flat as well, with palm facing down, and move your hand in small circles under your non-dominant hand, as if you are drawing attention to the space below the "floor".
Teach your child the bottom sign in conjunction with the other signs of location or prepositions. Then use these signs in hide-and-seek types of games with your toddlers.
You can play a modified "hide and seek" game involving your toddler's toys and other items. Use the sign flash cards for prepositions and other location words to give your toddler a clue where he or she can find the hidden toy in the house or room.