First, you have to pick your flowers. The best place I can think of to buy fake flowers is at craft stores, Michael's being my persona favorite with Hobby Lobby coming in close second. Online, I've heard good things about Afloral, but have yet to check them out. Flowers tend to come in a variety of materials (and prices), from hyper-realistic silk to obviously fake plastic. You can still make a very pretty flower arrangement with fake-looking flowers, so just buy what you can afford.
When choosing flowers try to keep in mind your rooms personal style and don't just go running for the first black blooms you see. My favorite fake flowers for home decor are roses, dahlias, peonies, baby's breath, and ivy, but there are flowers for any aesthetic. Daisies (painted red, black, and white, maybe?) would go cute with a retro-aesthetic and lilies (especially white) are great for a romantic look. Around Halloween time you can always find a variety of dark false flowers, especially black ones and white roses with "blood" splatters. If you can't find flowers in a color you like, it's not particularly difficult to spray paint them your preferred shades.
Next, you have to find a container. Vases are traditionally used but I love seeing flowers arranged in larger bowls, in jars, in perfume bottles, in dinnerware, even in tea pots (such as the above display, which is in my house's entrance way.) For your Goth sensibilities, try miniature coffins, absinthe bottles, candle holders, or other such containers. When choosing your container, it's worth remembering that having a display where the flowers and vase are the same color makes for a very drab arrangement. Vary it up with texture or color to make a more interesting display.
After you have selected your flowers and your display, you have to arrange your flowers pleasingly. Generally speaking, it's advised that taller flowers go in the center of the display and smaller flowers around the outside so that the smaller flowers don't get overwhelmed. If you have trouble getting your flowers to stay put, fill the container with pebbles or foam to stick the flowers in.
Lastly, you're not just limited in putting flowers into your arrangement. Other decorating elements, accessories to the arrangement, can be used to add more Goth style and to help the flowers match the rest of your decor. Accessories like ribbon, feathers, fake spider webs, jewelry, and anything else sparkly will really bring the arrangement to a whole new level of spook.
Wait, did I say lastly? Well, that's not entirely true. Once you've found a spot to put your lovely little floral piece, you need to upkeep it. Fake flowers are dust magnets and while this can add a certain spooky charm to it, it'll be a nightmare for someone with dust allergies (ahem, me, ahem.) Fake flowers can also bleach if left in sunlight for too long, so respraying them or moving them out of sunlight would be helpful. It seems like an awful lot of work for fake flowers, but it's worth it to keep your display looking good as new, and so that you aren't stuck with a sneeze-inducing, bland decor accessory.
Here's the floral arrangement I made just for this post:
Not bad, huh?
Do any of you decorate with fake flowers? What flowers did you use, and where are they? Or do you keep live plants?