Most charming community member 'Sunnylady' has already written an article about how to make liquid white and liquid clear. Wonderful!
With this article, I want to slightly describe how to make liquid black. Of course you can purchase a pot of liquid black in any descent artist store or even order it online, no worries. But liquid black is in fact not very common used, so it might be far more economical to just make it yourself whenever you need some!
Two things are needed for this mix, namely some ivory black (or midnight black which is Bob Ross's own mix) and some linseed oil.
Take about equal amounts (50-50) of both and mix them together. You can use a spatel/knife but also a brush will do the trick. Make sure it is evenly mixed and then apply it to the canvas. If you feel it is not smooth enough, add a little more linseed oil. If it is way too smooth, just add some more paint. Don't worry, this is really an easy mix to do. Enjoy!