First I use Final Fight on MAME, and a screen grabber, I use Greenshot.
1. Play/record the game and take screenhots. Some of the background will be obscured by other objects, scrolling in small increments and taking shots as necessary (and after a bit of guesswork and close editing) a complete picture is gradually made up. This one is a current wip
Using paint shop pro 7, the screen dimensions are shrunk by 77w by 89y percent and recoloured to the games palette. The recolouring is done by reducing to 256 colours, and then reducing by as much as possible down to 32 or less colours. The first stage uses 24 for the background. I fine tune it by making similar colours match to edit out some colours as I go and keep the shades I want in there. Eventually ending up with something like this:
If I can get help doing just some of that, it would save me at least a week in total per stage, and I can concentrate on making it go.
2. If the backgrounds are small (less than 2 screens width like above) they can probably be left at this stage. Otherwise they are mapped.
The background is made into tiles of 16 * 16...
...and processed by order into a small image for ripping into the game.
This is the most time consuming process.
@turrican3 and all: The answer to how long this will take to complete depends on the time it takes to do this in a big way.
Bobs are slightly more simple. Usually there is a good resource out there with most of the original frames ripped already. The resizing is the same, and the re-colouring process is the same. There is no need for mapping.
I'll put all the graphics I've got into a file for downloading a bit later