Black magic spell casting isn't all that uniquely different from any other kind of spell casting or magickal working, but there are a few things that you should know before you start.
The first point is that you shouldn't let your emotions take over. Yes, your anger can be a very powerful source of energy and shouldn't be left out of your spell, just don't go rushing into any spellwork while seeing red. You will have poor focus and nothing will get accomplished. If the situation is a really volatile one, you should let yourself simmer down before getting out your hexing tools.Next, make sure this is the right type of spell. A black magic spell isn't somehow "stronger" or "more powerful" than a white one, at least not usually. Yes, it's much easier to build up your personal energy when you have a big hate on but that's not really about the spell then, it's about you.
In many cases, a white spell would be just as good and less hazardous to your personal karma. Want to find love? Black magic is great for that, but so is white magick. Anyway, you get the idea. Check out the various spells and try to find the most suitable one regardless of the orientation.
As I've mentioned before, black magick isn't all about summoning demons or evil spirits. It's just magick like anything else. So don't get all confused when your spell doesn't have you chatting with Lucifer or Baal. Well, some spells might, but that's another story. You'll mostly likely being using the same types of natural items you would use in any other spell, without a lot of strangely exotic things like black bat bones. That's movie-magick talking.
Anyway, my last tip for black magic spell casting is that you don't go overboard. Basically, once a spell has been cast, you should end it or dismantle it once it's done its job. You may like the idea of someone being plagued by bad luck or nightmares forever, but these types of spells are better done for a single purpose. Once that purpose has been completed, let it go.
So as you embark on your path to darker spells, try to keeps these thoughts in mind. It will help you be more successful with your spells, and to further understand how magick works.