Pants is signed by brushing either side of your pants (or the normal pants' pocket area of your hips) simultaneously with your open hands that curve and straighten out as they brush your pants.
Another ASL sign for pants is to use your flat open hands to indicate one leg, and then the other, by holding your parallel hands vertically as they move downward from your upper to middle thigh, first in front of one leg, then the other.
It's useful to teach your baby the sign for pants along with other clothing signs. This not only teaches them to anticipate when you're going to get dressed to head out. It also helps them get excited knowing they're going out with Mommy or Daddy soon.
Teach this sign along with the other clothing signs. As your baby gets older, they may be happy to be able to tell you what they would like to wear before you head out.