In the very beginning of Goth culture, DIY was a very important element of fashion as it was almost impossible (not to mention terminally boring) to just go out and buy pre-made, pre-packaged outfits. But during the 90s and early 00s, Goth and alt fashion was determined as a viable market, hence the boom in shops and name brands (Lip Service, Hell Bunny, Poizen Industries, Living Dead Souls, Dead Threads) selling ready-made clothes for alternative types. For a decade at least, it became the norm, particularly amongst younger Goths, to simply stroll down to one's local Hot Topic (USA) or other self-proclaimed alt fashion store and purchase outfits that were not too far from being exactly the same as every other Goth's outfit.
This is not to say that there is something 'wrong' with wearing off-the-rack Goth fashion - I have a fondness for certain brands, including those listed above, and certainly it's a lot easier than crafting every individual outfit from scratch, especially for those of us with limited skills. Also, many mainstream stores stock pieces that can easily be co-opted into a Gothy outfit.
But as many of us are as vain as peacocks (in my case, vainer), and most Goths strive for individuality in their clothing choices for even the most mundane occasions. Customising your clothes is the best way to give each outfit a hint of your personal style and flair, and can turn a plain shirt, jacket, or pair of jeans into a visual masterpiece worthy of the most dramatic Gothic diva.
Paint It Black
Break out the black dye and fabric paint - dipping something in darkness is the quickest way to Gothify a wardrobe piece. Be very careful to read the instructions and make sure that the dye you've bought will work on the type of fabric you've chosen. Most light-coloured T-shirts are dye-able, as is pretty much any item made out of pale-coloured cotton. You may want to test your dye on a cheap item or rag first to avoid wrecking a nice garment. Don't forget, black dye is also great for home decor - let loose on curtains, bedsheets, rugs, tablecloths, etc.
Jeans can be slashed up the thighs or torn at the knees. Skirts and sleeves can be frayed (use a cheese grater) or cut at the hem to make them jagged. T-shirts can be torn and ripped. And never be afraid of holes in your tights!
Put some fishnet on it
Take one pair of fishnet tights, rip a hole in the crotch, and pull over your head like a T-shirt. Et voila, instant fishnet sleeves. Add layers of fishnet in different colours, or layer on different-coloured opaque tights.
You can buy band patches, logo patches, cult movie patches, patches with pretty much anything on... with just a simple Google search. Or you can cut designs and logos out of old T-shirts or hoodies, or paint, embroider or screen-print your own patches. These can then be ironed, safety-pinned or sewn onto anything you like.
Safety pins are your friends
Put rows of them down seams. Pull a garment apart and 'stitch' it back together with pins. Use them to hold patches. Put them on your lapels.
If you're the artistic type (which wouldn't surprise me), you can get fabric paints and splash your own designs onto clothing (or cushions, curtains etc.).
Put studs on it
You can buy packs of spikes or studs online (or in some alternative shops) and use them to decorate, again, pretty much anything. They look especially good on jackets, jeans and bags, but if you're looking to makeover your bedroom you could wrap photo frames in studded fabric, or do designs on cushions using spikes (although that isn't a practical or comfortable idea!).
You can get badges (or buttons, for my army of readers in the USA) almost anywhere online (or, again, in most alternative shops). Badges featuring Gothic artwork. Badges with slogans, band names, Siouxsie Sioux eyes... you name it. Get Goth slogan badges at this link or demonstrate your loyalty to real Goth music with badges from Batcave Benders.
Your needlework technique doesn't need to be perfect for you to to sew a design onto a cushion, T-shirt, bag, thin jacket (I wouldn't bother attempting to embroider leather or PVC) or pair of gloves. Practise on some scrap fabric or even a piece of paper until you get the hang of it. I like to sew words onto the knuckles of fingerless gloves - you know, like those LOVE/HATE tattoos.
Listening to: Close to Me - The Cure