Just as there are many ways to skin a cat there are many ways to etch metal. The most common one is using acids, which for instance is very common in PCB manufacturing to eat away the copper on the circuit boards. This, however, requires handling of corrosive chemicals which oftentimes also cannot be disposed of safely. While completely doable even in a home setting (look up copper/brass etching with ferric chloride for instance) there are better ways to do it, which requires absolutely no dangerous chemicals.
What we need:
Metal, one piece which you want to etch and a sacrificial piece of the same material
Salt, sodium chloride, but any salt which makes water able to conduct electricity should work
Power supply, preferably something which at least could churn out a couple of amps, but in a pinch even a 9V battery works
Resist, as in something which you can cover the metal with to selectively choose where to etch, can be plastic tape or even sharpie marks
A cutting tool, in the video we use a laser cutter, but there's no reason that an xacto knife and a steady hand couldn't be used as well.
And that's it, using these things you can easily utilize the method described in the video above.
Warning and disclaimer
Electricity and water can be a dangerous combination, especially when working with higher currents. Be careful and make sure not to (as I did accidentally in the video) touch both electrodes at once with bare hands
Depending on the material you etch you may release particles into the water which are not environmentally safe, and in some cases may not be healthy for you either. Stainless steel for instance may release hexavalent chromium which is a listed carcinogen, and when etching brass or copper you will release copper into the water which also needs to be disposed of safely (i.e. not flushed down the drain). Look at the alloy mix of the metal you're etching and do your research if it's environmentally safe or not.
Also, make sure that you do your etching outside or in a well ventilated area as it may release anything from minute to decent amount of chloride gas, which can be immediately dangeous to human health as well. With fair ventilation that risk is however removed.
Hope you enjoy the video, and make sure to subscribe on youtube if you want to keep updated with future updates, as all may not be posted on instructables.
Thank you, and happy etching!