What I do is this: I open up Dwarf Therapist, and sort the dwarves by highest skill. Dwarves with no moodable skill can be of any skill level, of course, but more of them have few or low-level skills.
Then, look for ones without any skill in Mining, Carpentry, Crossbow-making, Masonry, Engraving, Tanning, Weaponsmithing, Armoring, Blacksmithing, Metalcrafting, Gem Cutting, Gem Setting, Leatherworking, Woodcrafting, Stonecrafting, Bone Carving, Glassmaking, Weaving, Clothesmaking, or Mechanics. (Those are the moodable skills)
Choose what type of item you want: if it's furniture, then you want to train them as masons, if it's artifact weapons, train them as weaponsmiths, if it's armor, train them as armorers.
To do the training, turn on that labor for the dwarves you've identified as having no moodable profession. Set one workshop profile (q over workshop and hit P, requires a manager noble) to accept only dabbling skill, and queue a job at that workshop. One of your trainees will take that job.
As soon as they've taken the job, turn off that labor (they'll still do the single job once they've taken it, even if you turn the labor off before they start doing the actual work). Be sure to not employ them in any other moodable profession, or you'll lose the benefits of the training, it's strictly something to do for haulers, farmers, wood burners, furnace operators, lye makers...anything not on the list above.