I have done some of this. It's fairly simple. I will say that I have never actually done it to the seat covers themselves because I would think that the possibility for failure of the "dye" would be greatest there due to the flexibility and wear and tear that the seat covers must endure - so I just replace the seat covers in the new desired color. Also, come to think of it, I have never used the stuff on a headliner. I have always just replaced the headliner itself with the new color since headliners are usually deteriorated and it's not worth wasting the "dye" on. The stuff may work fine on those parts as well but I cannot tell you that from personal experience.
The whole "dye" process really is just painting as you mentioned. You are actually just coating your parts in a special paint that is formulated to cover flexible parts specifically but can also be used on hard plastic surfaces and metal trim. Basically you can do your entire interior with the stuff. I've done my dash pad, the dash itself, the door panels, all the plastic kick panels and lower door panels, seat belts, console, metal rear window trim, just about everything in the interior with the exception of the seat covers and headliner as I mentioned.
SEM is the stuff you want to use. To dye your stuff black, you want "SEM Landau Black 15014" - That is the perfect color black.
I buy the stuff by the quart and spray it out using my small HVLP touch up paint gun. You CAN buy the stuff in a spray bomb can and just spray paint it on if you do not have a paint gun and compressor, but I do not know how well the spray paint works.
You will DEFINITELY need to thoroughly clean the parts you are going to paint. I use soapy water, wash the parts and scrub them with a red scotch-brite pad as I am washing. This scuffs up the surface slightly, giving the paint a better surface to stick to, and removes any greasy residue from the part which would resist the paint.
I recently did the interior of my car white. Although my car's interior was originally white, many of the original plastic parts were un-usable so I used a combination of left over black and green plastic interior pieces and just coated them with a custom mix of SEM white. The black pieces (dash, console, seat belts, etc) were done in the SEM Landau Black that I recommended.
This is a pic of the pieces that make up the majority of my interior...
And here is what it all looks like after I used the SEM "dye" paint
Those pics aren't the best but I can tell you that, in person, it looks really good and I feel fine recommending it to you.
