This is a question I answered on Quora. The original is here.
This will depend heavily on what you mean by ‘macabre’ art. For the sake of this answer, I’m going to assume you mean dark subject matter like one would find on heavy metal t-shirts, horror-themed comic books, or the imagery I see when I do a Google image search on ‘macabre art.’
Art like this usually depends on taking normal subject matter and giving it a dark twist. So you’ll want to learn to draw basic people and animals well. Human skulls, human skeletons, and animal skulls and skeletons are easy to make creepy, so learning to draw these are also solid starting points for macabre ideas and imagery.
A great way to begin this process is to get a plastic skull or skeletal and anatomic models of humans and animals and draw them a lot, from many different angles.
Another element that sets the stage for macabre imagery is the overall scene and style. To practice or learn this, find a handful of high-quality images (either in books or online) of other artworks you really like. Then do a drawing copy of that artwork. Don’t worry if your copy is not perfect or if does not exactly replicate the original image. When you do a copy, the process of doing it will feed your artistic consciousness and you’ll be able to take elements of what you pick up in doing the copy and apply it to create your own imagery.
On my own website, I have a tutorial posted on how to do a master copy. It’s geared towards learning from old master drawings, but you can use this same process for copying and absorbing artistic lessons from any kind of imagery: How to do a Master Copy Drawing
UPDATE: See this post for great resources for pictures of bird and animal skulls.