First, clear out a (about) a 150 block wide and about 50 block high box. Then, at the bottom, place a 101 block long line of mud blocks 2 blocks above the ground. Plant mushroom grass seeds on the mud blocks. At the top, make a box of any blocks. In there place a water candle. Then, check on the makeshift mushroom biome (THIS ALL MUST BE CAVERN LEVEL) and you can see at least one truffle worm ( if not, try again) there. Hope this helps?