Change the Color
When looking at the basic outline of your DIY inspiration, be it a scrapbooking page, a sewing pattern, a miniature, etc. imagine it swapped into different colors. Black is an obvious choice but add jewel-toned accents, especially red and purple, and small shocks of white. Swapping anything bronze or gold for silver will also more easily identify it as a Goth piece.
Change the Material
If you're sewing, keep in mind that you can render your own version in more Goth materials to give it your own flavor. This is especially true if you're looking at clothing patterns. Swap basic knits and cottons for velvet or leather or add overlays of fishnet and lace to turn something normal into something Siouxsie-worthy.
Change the Motifs
Where the craft inspiration might be full of daisies and baby animals, you can add a gothic edge by swapping these motifs for something spookier. Bats are an obvious choice for many of us, but also consider spider webs, coffins, crosses, ankhs, moons, skulls, tombstones, gothic architecture elements, corset lacing, pirate skull and crossbones, and other things that are more easily identifiable as Gothic.
Readers, how do you usually add a Goth edge to your DIY projects when the inspiration is more or less plain?