The inked areas will remain in relief. This is important if you are using letters or numbers. Remember to reverse the design. Hold design up to a mirror to check everything is the right way around. Clear Cellotape or Brown packing tape will resist Etchant and lasts longer than the ink.
Tape is good for sharp-edge geometric shapes.
Stick tape to a piece of glass and cut out shapes with an Exacto knife. Gently lift tape off glass with tweezers and press down onto the metal. Protect the back and edges of metal from being etched by applying Tan Tape to the back and edges of metal. This saves a lot of pen ink. Put etchant in glass or plastic container.