Doesn't really work now. Mushroom farming was killed with 1.8. If you want to try you need to make sure you only place one mushroom every 8 or 9 blocks apart, even above each other it needs to be that many blocks apart. I think the number is eight but be safe and do nine. They spread even slower than they did before so if you need them fast just use bone meal one and chop down the giant shrooms to get more little ones.
How do you get mushrooms to spread in Minecraft now? I'm currently at 1.8.1. Are there any videos for it, or can you yourself give me some help?
Mushroom spreading has been restricted since 1.8 to 5 mushrooms per 9x9x3 area, but mushrooms can be multiplied quickly and easily using bone meal: place a mushroom (at night, in the rain, or in a large cave to avoid full sunlight), and put bone meal on it to make it grow giant. Then break the giant mushroom to harvest little mushrooms from its blocks. More info on mushrooms is available on the wiki.