I created most of my BJD stuff for two reasons:
1. It was fun to try,
2. I couldn't find what I wanted.

Brass etching falls into 2nd category. I wanted a triangular brooch with Celtic patterns, in some golden metal, and small enough.
After a thorough search on Aliexpress I ended up making it myself.

Tools and materials you'll need:

1. A sheet of brass.
The sheet should be not too thin so it won't be bent too easily, and there is enough thickness for etching, as it makes metal thinner. It also shouldn't be too thick, as the thicker the sheet is - the harder to cut it.
I use 0,5 - 1 mm thick sheets.

2. Alcohol based / permanent marker, for drawing the patterns on metal. Because water based markers are easily smeared by your fingers.

3. Scissors for cutting the brass. I use a pair of heavy kitchen scissors, with thick solid blades.

4. Tools for evening out the edges of the brass piece. It's hard to make clean cuts with scissors, especially if there are inner angles or curved edges, so you might even need a drill to make holes, and a selection of rasps / files to remove the burrs and protrusions.

5. The varnish for metal protection. It can be any nail polish, and my advice is to use a color that differs much from both the brass and the marker.
You can try water based varnishes, like colored wood varnishes, to avoid the smell.

6. The knife to scrape off the layer of varnish, revealing the areas that will be etched.
P.S. If you can paint the desired patterns with nail polish and thin brush, you don't need to scrape anything, just make sure that the layer of varnish is solid enough.

7. The concoction that would etch the brass, and a glass jar prone to chemicals.
Concoction recipe:
- 100 ml of 3% hydrogen peroxide (can be purchased in apothecary / drugstore),
- 30 grams of lemon acid (usually used for baking, can be purchased in grocery store),
- 5 grams of salt (I believe you already have it at home).
I made 1/4 of that amount for my tiny brooch.

8. Ammonia liquid, cotton disc and the glass jar, tight enough to contain all the fumes inside, for making patina.
10% Ammonia can be purchased in apothecary / drugstore.

9. Fine sandpaper (1500-2000 grit), or piece of felt, or magic block, to polish the piece after applying the patina.

The same kind of etching can be done to copper as well, and the recipe that I found was intended for copper firsthand.

Here we go.
First of all I drew the desired brooch on paper, because correcting any mistakes right on brass is much harder.

Then I outlined the shape with permanent marker.

Cut the brooch with kitchen scissors.

My rasp and scissors (I have several similar scissors for cutting metal):

Covered it with red nail polish. Both sides and all the edges, thoroughly!

Drew the patterns with marker (black lines), then started to scrape off the nail polish layer with a knife (bare brass in gold lines).

Finished: all bare areas will be etched.

Put the brooch into the concoction and wait.
I recommend to leave it for an hour, then to retrieve the piece and see how much of brass was etched, so you know how fast the reaction goes. Then you can decide for how many more hours to leave it for desired depth of etching.
I'd check it from time to time anyway, because there might be holes in protection varnish layer, so you should notice it in time and add more varnish if there are any bald unwanted spots.

IMPORTANT: please be careful around the etching concoction and wear gloves or use tweezers to take the brooch out, as it's an acid and can't affect your skin!
I didn't notice any fumes though, but I covered the jar anyway.

Etched and rinsed. As you can see, etched area is matte, which is good for contrast.

I polished the piece with sandpaper...

It's shiny and pretty nice.

But I decided that it's not enough and I want more contrast.
So the piece goes into another glass jar for patination.
I drilled two holes in the brooch, so I could put a thread through those.
Then I put a cotton disc to the bottom of the jar and splashed a bit of ammonia liquid onto it.
I could splash it right into the jar, but it would spread unevenly to the sides of the bottom, so I prefer to use cotton disc, as it helps channeling fumes where I want them.

This chemical reaction can be sped up with heat, so putting your jar into a warm place or near a source of heat is a good idea.
Also check the piece every hour or so, to see how reaction goes.

IMPORTANT: Ammonia solution is extremely smelly, can affect your skin and respiratory tract, so put the brooch into the jar and retrieve it from the jar in a well ventilated area, wearing gloves and full respiratory protection! You can do it FAST, holding your breath, but I prefer to use organic vapors respirator from 3M.

Patinated piece has a nice dark color in the etched area, and the protruding area dulled just up a bit (I suspect that my brass sheet was protected with something but can't be sure).

I polished protruding areas with sandpaper again. Shiny!

Attached to the clothes. The photo is made more than a year later, so the surface dulled a bit due to natural oxidation.

You can protect the brooch with transparent varnish, so it stays shiny.