Meat Hook is your most important spell as it is the beginning of the combo, and it is what setups for your Dismember and Rot, which are both melee range.
Mechanically, the skill is quite simple to understand: it damage enemies, drags them towards you and disables them why they are being dragged.
Positioning & Vision:
When it comes to landing the Hooks, the first thing you must think about is your positioning: try to Hook from an angle that doesn’t allow creeps or ally heroes to block your Hook.
Vision is also one of your top priorities. It is much harder to dodge a Hook if you don’t expect it. Players wouldn’t have quick enough reactions to do it intentionally, so by far the best way to ensure you are going to land a hook is to make yourself unseen. This usually happens by using the fog of war and obstacles. Stay on high grounds or behind objects (like cliffs and trees) so that your target doesn’t see you. This is not only good for the shot itself - when you pull the target behind an obstacle, it will be much more difficult for it to run back to a safe position, or for its allies to help out.
Sometimes you might have the chance to Hook a hero which is surrounded by allies on a high ground, and because of your good positioning, the allies wouldn’t be able to counter initiate and save their teammate.
You can place Observer Wards in key places to provide vision for your Hooks. Moreover, you can use a Smoke even only on yourself to make yourself invisible for enemy wards. This will make it much easier to land your Meat Hooks since enemies will not spot your rotation even if they feel safe because they have Observers. Keep in mind that on max lvl you can use Meat Hook from a high enough range in order not to break the Smoke effect. This will allow you to initiate out of vision even if you are not hiding in a position where you are under the cover of Fog of War.
Anticipation:
You must bear in mind that it takes time for the hook to reach from you to your target - the projectile travels with a speed of 1600, so from you to the max range of 1300 it would take 0.8 seconds for the Hook to hit. This means that clicking on the target, especially when it is away from you, is probably going to miss if the target moves. Therefore, you must anticipate how the target will move.
Usually, heroes move in a straight line when they are moving from one point to another, so all you have to do is use the skill a bit to the front of the target (the further away you are from it, the further ahead of it you must aim at). If the target is standing still (channeling a spell, auto attacking and not moving), you should click on the actual target.
When your target sees you and anticipates the Hook, it becomes a bit more complicated. Most probably the player will try to wait to see that you are casting the Hook and just at that moment will move a bit to the side in one direction to dodge the skill. This means that you will probably miss if you aim at the actual target or at the straight line path of movement. You should anticipate the actual dodging movement. As a result, it is best to use the hook a bit to the side of the target. This way if the target dodges (generally 50% chance), you will successfully hit. Provided you are not certain how the target is going to dodge, you can cancel the Hook animation just as you are throwing it (by pressing S right away). This way your target will try to avoid anyway because it sees your casting animation, and you might use this split-second advantage to cast the Hook again and hopefully land. This practice is very useful especially when you are casting in a short or medium range (the player will have to dodge right away; from max range, the player has the luxury to wait and see the actual projectile and dodge afterward).
Save Allies:
As far as the use of Meat Hook goes, remember that it is not only a tool for initiation. You can save allies by pulling enemy heroes away from them, or by pulling them towards a safer position. However, be careful not to pull them by mistake – if you pull your carry while he is focusing an important enemy hero, this might cost you the fight.
Finally, don’t be afraid of missing your Hook. Luck is always involved, and you can never predict the movement of your opponents 100% - it is much better to try multiple hooks, miss some but land some rather than try just a few when you are 100% sure that you are going to hit. Even difficult shots are worth trying most of the time.