To sign triangle, form both hands into fists with your index fingers sticking out. Starting at the apex, hold up both pointer fingers in front of your face, then bring them down in unison to form the acute side angles of the triangle. Finally, have them form the base by meeting at the center bottom of the triangle.
Teach your toddler the triangle sign when introducing them to basic geometrical figures.
The flash cards a are a great medium to teach the basic shapes to your baby. Once they learn to make the association between triangle and the flash card diagram, try and help them broaden their understanding by finding triangles in your environment. For example, they may see triangles in roof tops or Christmas trees.