After Just achieving rank 1 windmill myself (took me like 4 days) I can say that this is probably the fastest method possible for a new-from-scratch player.
I am going to break down the grinding by rank and what you should do:
Before Anything kill some foxes for the first couple ranks of windmill (F, E, or D around) take up some of the goddess missions until you get the beginner adventuring gear (I.E. the broadsword, armor, etc.) go ahead and buy another broadsword and go learn the heal skill and the first aid skill (you will need these, train them to F and keep them there) NOTE: DO NOT TRAIN ANY OTHER SKILLS, IMPORTANT.
Anyway, on to the windmill guide itself
Rank D (ish) --> Rank 7 or 6
For this you should buy a lute, equip it, and you should get a quest to head on over to the bard camp. From there you will talk to Yvona and proceed to the instance to kill the bandits, from here DO NOT COMPLETE THE QUEST (I.E. do not use discord on the enemies) instead, just windmill them over and over, for the giant boss I usually put up my broadswords and punch him for one combo then windmill, its easier that way. Once you hit around 7 or 6 you can stop, finish the quest (because you want that stamina recovery enduring melody) and move on with your life.
*** Note, for killing multiple enemies you will want to create a party> set the options to anyone in party under the kill option I believe. This will enable it so that when you should kill an enemy instead it will be lying on the ground for you to finish off (This is where you windmill it).
***** Do not use the party option with the bard camp quest you should be able to one hit the bandits.
Rank 7 or 6--> Rank 1
From here finish up those goddess quests until you get to tir na nog, and I mean do them like you are mad (it took me probably 4-5 hours of my eyes glued to the screen and my warp book on fire) (While you are waiting for Tarlach at Night By The Way head over to the logging camp and kill bears with the party method mentioned above, they will be boss and are fairly easy to kill, just use counter attack and windmill on repeat until you can finish them with windmill...)
Once you get to Tir Na Nog you will want to get used to fighting the zombies in the graveyard, as they are going to be your main source of experience besides Their Method (more about this in a minute)
Go ahead and look at your windmill exp and see what you need as far as boss kills (always take into consideration the multi kills/strong hits before you calculate the boss kills required) then proceed to the graveyard to kill zombies. At this point you should turn party off and just proceed to ONLY USE WINDMILL to fight them. They are really slow and have low hp but have passive defense so any normal attacks have a chance to be countered/blocked, so just windmill.
Once you have a handle on the zombies and you killed the ones you need (if any) for the boss exp. you need for that level then warp on over to Dunbarton and check to see if you can buy some min-max broadswords (that are better than the ones you have) from people, just spam you want to buy broadswords on the trade channel or look through shops.
Once you have your newly improved swords (should be about 25k a piece btw). You are going to want to buy a control bar for a very important skill for the next part (not entirely necessary but it helps sometimes). You just equip the control bar and you magically get the skill you need, wire pull. You can use this to pull mobs towards you (useful for corpse piling positioning).
After, Port over to Tara and head on over to the shadow mission board. From there you are going to go to Fledgling and look for the Their Method Quest. Take it, head on over to the stone and go into the instance. make sure your party is set to on. Here you will see groups of 4 zombies that should be strong level for you at this point (check with ctrl. If they arent you can always upgrade your combat mastery one or two times, but they should be). Now you check your windmill skill again, see how many groups of strong enemies you have to kill, and kill the zombies , using your control bar if need be but you should be proficient enough in windmill at this point to knock them into a group close enough that you can kill them all at once, making sure to hit all 4. Keep doing this, running through the city until you cant find anymore (usually about 4-5 groups per instance). Then EXIT THE MISSION. You don't want to complete it as it is one, a waste of time, two, gives you too much exp we want that CP down, and three completing the mission counts towards your daily, if you exit you can take it over and over again killing as many zombies as you want.
Now, if you need to do awful kills for ranking up, you can always go to intermediate and do the same mission and kill the zombies there, or simply port over to the bard camp and kill the pigs around there, they should be awful.
At this point you go back and forth to tir na nog and and Tara, where it should look something like this:
Boss kills complete, rank up windmill, do boss kills you need (to save a trip), then port over to Tara
At Tara do multi kills you need, rank up windmill, do multi kills you need (to save a trip), then go back to tir na nog.
This will get you to 1, is really simple and in my opinion among the fastest options available.
That Is pretty much it, if you have questions please comment, and I am going to put some more tips further down for any general questions I think might come up
*Rank 4 windmill now requires you to only do strong multi kills and hits, you can get this whole rank in about 10-20 minutes of doing Their Method.
*To return to Tir Na Nog equip the golem slayer title you should have, Talk to Kristen and use the go to Tir Na Nog keyword to get a pass. You can get multiple by dropping the pass, talking to her, and using the same keyword. You really only need like 5 or 6. then go to the barri dungeon in bangor and go through the same dungeon you went through before (should be fairly easy for you now).