^ 99% of it is practise

i dont think theres a way to "learn" multis

most people will never get a good grasp on it, but they arent ESSENTIAL if u know what i mean... at least, try and have 2-syllable rhymes... if your vocab and meaning are good enough, there wont be a prob... but if you can, by all means, use as many as u can... the best ive seen in rap so far is an 8-syllable rhyme... i personally can do about 3 on average throughout a piece...at first i couldnt do any, but with practice (write write & write) you should be gradually getting a 2 syllable flow and it will progress the more you write... sometimes u write multis without even knowning it, and when that starts, your prolly on your way towards grasping them... but always TRY to use them - not at the expense of the meaning of a bar though... hope that helped, peace