Food processors are great for a lot of applications, and making almond butter is certainly one of them. It's best to work in small batches, utilizing the feed tube on the processor. Grind the almonds until they start to come together a little, then drizzle in a couple of tablespoons of oil with the processor still running. Stop the machine to scrape the sides down occasionally, and just process until the butter is the consistency you want.
If you want a really smooth almond butter, you can add a small amount of extra oil, just a few drops at a time. Otherwise, it will be too oily.