Building is signed exactly like build. Place your flat, dominant hand on top of your flat, non-dominant one - with both palms facing down - and begin a repetitive upward movement in which one hand supersedes the other until both hands are right in front of your nose. The movement conveys the act of building a tower.
Building can be used in combination with other toy signs once your baby is starting to sign phrases. For example, building blocks.
If you are getting ready to introduce the sign for building, you can do it using blocks or other building toys like Lego. You can also use the sign at the beach when building a sand castle.