To sign necktie, put your dominant hand's pointer and middle fingers (or just your pointer/index finger) at the knot area of your upper chest, and just trace it down all the way to the belly. Your non-dominant hand can be closed in a fist, with the same pointer and middle fingers sticking out, holding the knot area as a place holder.
Alternatively, you can draw the necktie with both index fingers in the shape of a bowtie.
We use this sign to develop familiarity with formal wear.
We learned to sign necktie when our babies discovered Daddy’s neckties.