Material

• Brother P-Touch that works for TZe tapes
  These printers work fine with the TSe tapes too - at least my experiece with the TP-P750W and the PT-2430PC.
• Brother stencil tape STe-141 (18 mm), STe-151 (34 mm) or TSe-161 (36 mm)
• highlighter
• Marking Head (AliExpresss ~12 CHF)
• Electrolyte (NaCl in water ;-)
• Oil to prevent rusting, when etching iron or regular steel.
• Power Supply 

Process

Print the stencil with Brothers P-Touch software.
Use the highlighter on the glossy side of the stencil to make to improve visibility.
Apply the stencil to the metal you want to mark/etch.
Use insulation tape to keep the stencil in place and protect the area around the stencil from being etched.
Attach the red (+) pol to the metal you want to mark/etch.
Attach the black (+) pol to the marking head.
Moist the marking head with the NaCl-sollution.
Turn the power supply on.
Swab the stencil with the marking head. You may check the current indicator to become an idea of the etching work done ;-)
Turn the power supply off.
Remove the power cords, stencil from the etched metal.
Clean the metal. 
When etching iron or "regular" steel apply oil to prevent rusting.

Result

I used for this stainless steel (1.4301) 9 V DC (constant voltage) ~10 seconds.
Etching/marking parameters vary from metal to metal.