Dwarf Foreman is essentially abandoned. As of this writing, Dwarf Therapist is the recommended replacement.
Download the latest version (0.3.8b) here. Source code is available at https://github.com/zorbathut/foreman.
Open a fortress. Open Foreman. You'll get a nice grid, as shown. Green squares mean "everyone with this profession is doing this job". Dark grey squares mean "nobody with this profession is doing this job". Light grey squares mean "some people with this profession are doing this job". Click on a square to toggle it between green and dark grey. Your dwarves' job assignments will be changed instantly.
For example, if you wanted to give all your Engraver dwarves the Stone Detailing labor, look for "Engraver" row by browsing the names on the left, find the "Stone Detailing" column by looking through the names on top (it will be in the white section), then click the square at the intersection of those until it turns bright green. Now all your engravers are engraving. Note that the above screenshot has a green square in exactly that location.
What do those buttons do?
"Load" scans Dwarf Fortress for new professions or manually-changed jobs and overwrites the current table with what it finds. "Write" writes the current tables to Dwarf Fortress, overwriting any changes you might have made (this is handy for immigrations - rename everyone appropriately, then click "write".)
Is it safe?
I think it's unlikely to corrupt Dwarf Fortress. I may be wrong. You use this program entirely at your own risk.
The chance of corruption is a bit lower if you pause before you muck about with this program. I've tried to make sure it won't screw up anything in Dwarf Fortress itself - at worst, it should crash - but there's always the chance of it breaking something horrible. I recommend making frequent backups of your fortress.
Foreman doesn't play nice with traders. It could, in theory, change their job assignments. I do not know what this would do to the game.
0.3.7b: Works with Dwarf Fortress v0.27.169.33c.
0.3.6b: Works with Dwarf Fortress v0.27.169.33b.
0.3.5b: Added all dwarf class IDs, improved debug output quite a bit.
0.3.4b: Now knows which flag is actually the death flag. More dwarf class IDs.
0.3.3b: Made it attach to a running Dwarf Fortress process in a much better fashion. More dwarf class IDs.
0.3.2b: Fixed some really, really stupid bugs. Also introduced a bunch of dwarf class IDs.
0.3.1b: Fixed the version check.
0.3.0b: Works with Dwarf Fortress v0.27.169.33a.
0.2.1b: Fixed fisher color, fixed ashery operating job, added auto-professions for dwarves without custom professions.
0.2.0b: Removed children, nobles, and military from display. Added dwarf color categorization. Added scrollbars.
0.1.2b: Added adamantine jobs.