The Sign's first appearance is in level 5, along with the Bat. There are two Signs, one readable and one an arrow.
For the ones with zigzag black dots (the readable Signs) you can read their text by clicking them. The arrow ones do not have text in them.
Signs can be obtained by crafting 4 Wood together with Red Ink for the arrow Signs and Black Ink for the readable Signs using a Carpenter's Workbench or by being inside of a Workshop.