This is how I etched a brass plate to use as a decorative plate for my laptop lid. I've also used these stuck onto the front of notebooks and sketchbooks as presents for friends. My method draws heavily from this instructable and this website, so I thank the respective authors for their sterling work. There are many, many different ways of doing this, but when I was researching it, there weren't very many thorough tutorials, so I think this may still be useful for some people. But if you're interested, do scour the internet and you'll find a wealth of information about toner transfer and etching - some good, some bad, some just plain puzzling.

The artwork I used is a piece called Tribal Eagle by *xx-trigrhappy-xx and is used with permission.

Step 1: What You Need (and Where You Can Get It in the UK)

You will need:

Brass plate (I got mine from eBay)
Iron
Computer to prepare artwork
Laser printer and laser OHP acetate (Or do what I did and get a print shop to photocopy it onto acetate for you)
Permanent OHP marker (useful for touching up the mask before etching)
Insulating (PVC) tape (Woolworths/Robert Dyas or hardware stores)
Masking tape (ditto)
Scissors
Non-metal kitchen scourer pad
Kitchen towel
Acetone (optional - but useful. Nail polish remover is mostly acetone and will do nicely)
(Hydrated) ferric chloride crystals ( Maplin)
Fine Wet & Dry paper (Wilkinson or hardware stores)
Some form of sanding block (I just used a scrap bit of wood I had lying around)
Cheap tupperware (You're not going to be able to reuse this for food!)
Rubber gloves (absolutely essential)
Goggles (essential if you're not stupid - don't risk your eyesight doing something like this!)
Dust mask (optional if you're careful and do the painting outdoors - there's not much painting involved)
Spray paint (I used black enamel satin-finish stuff from Wilkinson)
Superglue

Step 2: Preparing the Artwork

The first thing is to get your (black and white) artwork prepared and printed to the right size onto OHP acetate. This can be done directly by a laser printer, or indirectly by printing onto normal paper and then photocopying onto acetate. The process of toner transfer does effectively reflect the image though, so if you have text, make sure you mirror the image before printing. A word to the wise though: This has to be done with either a laser printer or a photocopier and not an inkjet! Only toner will mask off the areas of the brass that we don't want to mask - ink won't work.

Remember, any areas that are black will be masked off, not etched and will end up brass-coloured, whereas any areas that are white will be etched and end up black on the finished plate.

Step 3: Prepare the Plate

Cut the plate down to the correct size and shape and finish off the edges - it's easier to do this now than after etching. Most importantly though, clean the plate! Use the kitchen scourer to scrub the front of the plate until it's shiny (but it doesn't have to be polished) and free of any tarnish or dirt. Then, use the kitchen towel dipped in acetone to thoroughly clean the surface to remove any oil or grease. Try not to touch the face of the plate now - the cleaner the plate is, the better the toner will transfer later.

Mind out here - acetone is extremely flammable. If you get any on your hands, it'll dry out your skin, so use some moisturiser on it afterwards.

Step 4: Line Up the Mask

Align the plate with the artwork on the OHP film and use the masking tape to stick it down carefully. Make sure the tape is only on one side of the brass plate, as this will make removing the film later much easier. Also make sure the tape is taut, as otherwise the artwork will move and you'll transfer the toner in the wrong place.

Step 5: Toner Transfer

Place the brass/acetate sandwich on a heatproof surface (I used some scrap cardboard) and heat up the iron. You want the iron as hot as it will go and no steam! Start with the brass-side up and press down on the back of the metal with the hot iron. This will preheat the brass and help the toner melt. After 20 seconds or so, flip the brass/acetate over without touching the brass, which will by now be extremely hot. Place a piece of scrap paper over the acetate and once again, press down on the brass (this time through the paper and acetate) with the iron, keeping the iron moving. Be careful not to melt the acetate. The actual toner transfer happens very quickly, and I found it needed very little heating with this method - in the order of 5-10 seconds. Take the iron away and immediately peel the acetate back carefully - this must be done while the brass and toner is still hot (which is different to if you're using paper to transfer it). If you let the toner cool first, it will come away with the acetate rather than remaining on the brass.

You'll also find that the adhesive on the masking tape will melt with the heat, so it's easier to peel this off while the brass is hot. Don't burn yourself though!

Step 6: Check and Finish the Mask

This is the time to check that you're happy with the toner transfer. If you've messed up and need to start over, the acetone will dissolve the toner from the plate easily and you can try again (with a fresh sheet of OHP acetate though). If the toner layer is a bit thin, you can carefully align another printed sheet of acetate with your already partially-coated brass and try the toner transfer step again. This is one of the major benefits of using OHP film rather than the other options which people have said work (e.g. magazine paper, inkjet glossy paper, press 'n' peel toner transfer paper etc.). Make sure that the brass plate is free from bits of melted acetate, which can happen if you heat the acetate too much.
If you're happy with it, use the insulating tape to cover the back, sides, borders (if you want them) and any other large areas that you don't want to etch. Touch up any bits of the mask that look slightly thin on toner with the permanent OHP marker.

Step 7: Etch!

Time to etch! Put on your rubber gloves and goggles now! Make up the ferric chloride solution according to the back of the packet in your tupperware and immerse your plate in the solution. The etch will work much faster if it is warmed - I think the packet recommends 40-50 degrees C. I placed the tupperware containing the etch solution in a larger basing half-filled with hot water to achieve this. It will work at room temperature, but it'll take longer.
What I found much more important though, was to agitate the solution while etching. This ensures a fresh supply of etchant to the plate and will speed up the etch tremendously. I just donned my gloves and picked up the plate every 10-15 minutes and used that to swirl the solution around before putting the plate back down and leaving it again. Time-consuming, but it works well and has the benefit of simplicity.
My etch took about 3 hours until I decided it was done. You can monitor the progress of the etch by gently feeling the surface of the etch with your (gloved) fingers. You probably won't see any depth to the etch until it's nearly done. However, keep an eye on the mask, as eventually it will start to flake off - when this happens, you definitely need to remove the plate, as you'll start to etch in places you don't want to etch.

Ferric chloride is not terribly nice stuff, so do this outdoors and don't get it on your skin. Don't breathe the fumes and beware - it stains anything and everything indelibly.

Step 8: The Etched Plate!

No, we're not quite done yet. But here's a picture of the plate fresh out of the etchant bath. When you're happy with your plate, remove it from the etchant and rinse it thoroughly with water.

You'll see that my plate is covered with verdigris - this didn't happen on the first two plates I did, but it did on this one. I'm not entirely sure why...

Step 9: Disposing of the Etchant Solution

First thing - don't bother! You can keep the etchant for the next plate you do (it will gradually take longer to etch the plate as the solution becomes weaker though).

Second thing - when you do need to dispose of the etchant, do so responsibly. Don't just put it down the drain! The ferric chloride is corrosive to lots of metals, but more importantly, it now contains dissolved copper which is rather harmful to wildlife. What you need to do is neutralise the solution with an alkali (something like washing soda) which will precipitate out the copper as a solid. Then, you can dilute the liquid with lots of water and put that down the drain, as long as you filter/decant off the solid copper and dispose of that safely - your council will advise you how best to do this.

Step 10: Painting the Plate

Time to finish it off! Once again, clean and dry is the name of the game here. Get hold of your spray paint and shake it to mix it. Do some test spraying onto old newspaper until you're happy with your technique - remember, light, even passes larger than the object being sprayed, and thin coats. Once you're happy, stick the plate down and paint it! I sprayed mine with three light coats of the paint, letting it dry between each coat. Remember to spray from different angles to ensure that you catch all edges of the etched plate. Ensure the paint is thoroughly dry before continuing - if you're impatient like me, use a hairdryer!

Remove all the tape from the edges and back, and you'll probably find that it's left all sorts of gunk. I'd clean this off carefully at this point by scrubbing gently. Rinse the plate and dry with some kitchen towel.

Step 11: Finishing Off the Plate

Nearly there now. Grab your Wet & Dry paper and wrap it around your sanding block (the block is just there to keep the paper flat) and sand down the surface of the plate. This will expose the brass on any areas that were not etched. Don't worry about the way it scratches the paint surface - it gives it a nice pseudo-aged look.
Once you've cleaned down to the brass all over the unetched areas, add a bit of water to the plate (this makes the Wet & Dry effectively a finer abrasive) and scrub with small circular motions all over the plate to ensure a nice finish. If the paint has been scratched unevenly, then carefully use the paper without the sanding block on the unscratched areas and even it out. When you're happy with the finish, rinse the plate and dry.

Don't forget, if you completely screw up this stage, all is not lost - just clean the plate, respray and try again!

Step 12: Attach to the Object of Your Choice

Final step - glue it on! I used superglue, as it's quite good at sticking mixed materials (in this case, metal to plastic) and can be removed with the appropriate solvent. I'd recommend using the masking tape as before to allow you to align the plate first, flip it up so that you can spread the superglue and then flip it back to stick it correctly in position - superglue dries really quickly!