You wish to craft a +2 longbow. First, we must figure out the longbow's Caster Level. We find this under Magic Weapons.
For an item with only an enhancement bonus and no other abilities, the caster level is three times the enhancement bonus.
Which gives us a caster level of 3*2 = 6. You use your number of ranks in the skill you chose to link to your Master Craftsman feat as your caster level for purposes of crafting, so a 6th level Fighter can meet this requirement if he kept his ranks maxed.
Then, we find out the craft DC of the weapon. This we find under Magic Item Creation.
The DC to create a magic item is 5 + the caster level for the item.
Since our character is a Master Craftsman, this skill check is a check of that feat's connected skill. Its DC is 5 + 6 = 11. Since this bow only has an enhancement bonus, our calculations stop, and we have the final DC.
Had this been a more interesting magic item (one that requires specific spells as part of the casting process), this neat clause (again from Magic Item Creation is likely to be of assistance:
The DC to create a magic item increases by 5 for each prerequisite the caster does not meet. The only exception to this is the requisite item creation feat, which is mandatory.
Alternatively, our Master Craftsman could enlisted the help of others when crafting magic items with spell requirements, as described under "Requirements" in Magic Item Descriptions.