How to add a magical composite bow to a treasure haul?

Peregrin Took’s fine answer points to a perfect source, the Dungeon Master’s Guide example barbarian on pages 112-113, but I thought it might be worthwhile to point to an example available under the Open Game License, the solar:

+2 composite longbow (+5 Str bonus) +28 ranged (2d6+7/×3 plus slaying)

^{(Note: d20srd.org fails to italicize the “+2 composite longbow” section—this is an error with that website, and the official SRD .rtf files from Wizards of the Coast does italicize it.)}

On some level, this is the “Monster Manual format,” which differs slightly from the “Dungeon Master’s Guide format” for describing attacks—on the Dungeon Master’s Guide page 112 example barbarian, the attack bonus is listed first, and the weapon used last, inside the parentheses for the damage. None of that matters for describing loot, though—it’s just that the Monster Manual descriptions (and their SRD copies) don’t include “Possessions” like the Dungeon Master’s Guide example barbarian does on page 113.

The fact that this is available under the Open Game License may well matter for a good chunk of those interested in this question; after all, the primary reason to be concerned about how to unambiguously and consistently format these things is for publishing your own adventures, where you won’t be there the adjudicate or explain.

Also, the mere fact that the Dungeon Master’s Guide example and the Monster Manual example don’t perfectly match may be relevant—there is some wiggle room on the formatting here. I think it’s wise to include the enhancement bonus first—that much is very consistent across all manner of magic items—and to put the Strength bonus last. But you can vary it a bit according to your needs. Dropping the word “bonus” might save you a fair amount of ink and paper if printing a book, and I can’t imagine how that could cause any confusion.

You put the enhancement bonus in front and the strength rating in brackets behind. So it‘s a

+1 composite shortbow (+2 Str bonus)

You find this way of writing in the NPC statistics section of the DMG (for example in the stats of the sample 10th-level NPC barbarian on page 112/113).