Go into Modding Utilities subboard and download my Project: Modder's Resource.
Now to your Qs
I'm interested in doing what should (I think?) be something fairly simple - simply adding an extra option to an existing workshop (probably the Bowyers workshop) to be able to produce arrows. I'm looking at it requiring a reagent of, for example, wood and iron for iron arrows.
Saddly the Bowyers is off limits until toady makes it so, but you can use these: KILN, SMELTER, TANNER, KITCHEN, QUERN, MILLSTONE, STILL, CRAFTSMAN, SOAP_MAKER, SCREW_PRESS
I've read some modding guides on the wiki, but I just can't quite get my head around what files I would need to create and/or edit. I've looked through the raws and can't seem to find the existing reactions to smelt iron bolts; it was my hope to just duplicate this reaction, modifying it slightly as needed.
Best thing is to look at the raws in vanilla DF, however I can list off what you need:
Need to make a new Reaction (can make a new "reaction_xxxx.txt" or add it into an exsisting reaction_*.txt file)
Need to make a new building (unless you are just gonna use one from the list provided above)
Also you can't find a reaction on making arrows because the Forge (hardcoded) automatically makes those reactions with weapons/armor/ammo/etc and materials that say that they allow to make it (most weapon grade metal has [ITEMS_XXXX] tags) so you'll have to make your own. and that is not too hard
[REACTION:MAKE_BULLETS]
[NAME:make box of bullets]
[BUILDING:AMMO_REGEN:NONE]
[REAGENT:copper:150:BAR:NONE:INORGANIC:COPPER]
[REAGENT:lead:150:BAR:NONE:INORGANIC:LEAD]
[REAGENT:gunpowder:150:POWDER_MISC:NONE:NONE:NONE][REACTION_CLASS:GUNPOWDER]
[REAGENT:gunpowder container:1:NONE:NONE:NONE:NONE]
[CONTAINS:gunpowder]
[NOT_IMPROVED]
[PRESERVE_REAGENT]
[DOES_NOT_DETERMINE_PRODUCT_AMOUNT]
[PRODUCT:100:50:AMMO:ITEM_AMMO_BULLET:INORGANIC:COPPER_LEAD]
[SKILL:FORGE_WEAPON]
look at the [PRODUCT line at the end, and read up on the offline wiki for any quick things you need.
EDIT:
@laularukyrumo
I think they stack, Putnam will know more but I think I remember him saying that is DBZ guys where powering up so much that they went over the 4byte limit and went into the negatives (stack overflow)