Like ann poste start with that quets killing Rotface and Festergut on either 25 Hc or normal mode, and getting that hammer from dragonblight in a cave. after youve done that your going to need to capture souls, wich you do by hitting mobs in ICC with shadows edge, (a tip is to kill mobs, reset and kill more, doing this can allow you to get shadows edge + that quest , and continue onto infuisons on the same save). AFter all of that infusions quests start, youll have to start by doing Proffesor putricide normal or HC, but id recomend normal if u want a 100% certain shot to get it depending on your grp, while on boss you have to be the abomination, stack up your "energy" to 100, and then infuse the wep, DONE, next up is blood queen infusion, What i did here was i let 3 ppl in my raid rush in first, a tank and 2 dps, you want to be the third dps, person with the 3rd most threat, the third most threat gets bitten first, and after that you get the de buff and you will have to bite people 3 times to complete the infusion, Third and last infusion is sindragosa, what i did here was i stood infront of sindragosa hitting her with shadows edge until she breathed me and i got infused.
after youve done that youve completed the "hard part" of the shadowmourne quest line, the upcoming about 4-5 weeks after doing that you will be collecting shadow frost shards, wich can ONLY drop from heroic mode bosses, once youve collected 50 of these, and GZ to your smourne, you also recieve a quest to kill LK after that wich i would recomend doing seeing you will get smourne items, like the tabard and mount (Crimson deathcharger)
Sorry if this was alot and messy, but thats basicly the whole "How to get smourne" the fastest way right there