Check out the Tales of the Jedi comics for a better visual explanation...
But The Dark Lords of the Sith of the old Sith Empire wore tattoos of rank on their foreheads. The burning venom of a kind of pincerbug was used to burn the mark into the Dark Lords skin. Centuries later, the Sith Lord Naga Sadow would bear the same mark. 1000 year's after the fall of the Sith Empire, the spirit of the Dark Lord Marka Ragnos named Exar Kun the newest Dark Lord, and made Ulic Qel-Droma Kun's apprentice. Marked with Sith tattoos on their foreheads by Ragnos' hand, the newest members of the Sith joined forces. When Ulic returned to the light side, his tattoo was carved away, leaving an x-shaped scar. During the Jedi Civil War a few decades later, the fallen Jedi Knight Darth Revan founded a new Sith Empire and declared himself the new Dark Lord of the Sith. Several members of Revan's new Sith order sported tattoos. Uthar Wynn, the head of the Sith Academy on Korriban, and his apprentice Yuthura Ban had parts of their heads and necks adorned with a detailed pattern of tattoos. Revan's followers Darth Malak and Jorak Uln had their bald heads tattooed with a striped design.
By the time of the battle of Ruusan two and a half thousand years after Revan's time, the tattoos of the Sith had evolved into circles, wavy lines, straight lines, squares, spots, swirls, triangles etc. One example was the Sith Lord Githany, who bore a wavy, circular black tattoo on her forehead. Several Sith of this era had their eyelids and skin around their eyes tattooed. One prominent example was the Sith Lord Darth Bane, the man who reinvented the Sith order after its destruction and established the Rule of Two. Apparently, most of the Sith of this era wore tattoos on their head and/or face, though some had other parts of their body decorated. One of them was the Sith Lord Kas'im, who wore his tattoos on his chest.