I've wanted to make my own embossed business cards since I was handed a really incredible one at a fancy restaurant. I tried several methods of achieving the effect (photoemulsion, electrolytic etching) but ended up having the most success with a pretty simple and straightforward acid etch method similar to the one used in home printed circuit boards. I still have to work out a few kinks (such as flattening the paper after the emboss without distortion, or reorganizing the method to print before embossing) but I hope you can take something useful from my experiments, and apply it to your own projects.
Step 1: Planning
Access to a laser-jet printer
Ferric Chloride (available here)
An Iron (with all the water emptied, please)
A meticulously clean brass plate (big enough to cover the space you want etched with room for more)
Heavy, thick paper. Card stock will do nicely, but heavy rag paper from art supply stores works well, too.
A clamping jig to align your sheets each time you use it. I used plywood with brass elbows and PTEG plastic sheet (to keep the paper nice and flat against the clamping jig), but it's up to your expertise.
6 or more C-clamps.
Shiny laser jet paper.
A plastic bus bin, or other wide plastic container for the acid etch.
A spray bottle.
My concept was to emboss a sheet of paper, and then run it through a printer to color the page. I'd then cut the cards out. On review I'd have changed some things, but got some cool results, regardless.
Begin by designing the pattern that you want embossed in the sheet. Remember that what is embossed onto the card is the mirror image of what is etched on the plate. So if your pattern reads properly on the brass, it will be backwards on the card. The same applies to the relief. If your logo is raised on the brass, it will be inset in your card. Design it in sharp black and white. If you want gradated patterns, consider going with a halftone pattern.