I love using a thin leftover bar of soap to mark lines on dark fabric. It's a great alternative to using tailor's chalk. It's good for the environment as well as the pocketbook. Just make sure it's nice and dry before you start using it.
In the picture below, I'm marking a line on the stretch jersey to make a 1/2" band as part of the
finishing on a halter top. I bet you thought soap was just for washing !