Preparation
After reaching insight 99 (And possibly after Rededicating yourself), the next step in advancing yourself is Mastering. Becoming a Master means you have access to new skills, spells, equipment, quests, areas and more in the game. You also gain the ability to buy stats (Which will be explained more later). And further, you must Master before advancing to Gladiator, Bard, Druid, Archer or Summoner.
For each path, you need these things:
-Your Master item made (With two seperate items)
-200,000,000 Experience in the box
-Required skills Mastered or at a certain level
-Set Health and Mana requirements
-Be an Aisling of Talgonite, Hy-brasyl or Mythril.
So let's get started.

Aisling Recognition
Aisling of Talgonite: This is easily the most common type people choose, and is very simple to make. You must have 25 Finished Talos (Polish Talognites). If you're insight 97 or 98, you must sacrafice 50,000,000 experience. If 99, 100,000,000. It is really a matter of preference which you choose first. You may choose to do it at 97 or 98 for the low experience sacrafice, but some do it at 99 because they get to 99 without wasting a hunt, they get the advantages of being 99 (Higher equipment, Noes Citizenship, etc), and for some it's very easy to earn that 100,000,000 experience back.
Aisling of Hy-brasyl: This is one of the hardest to do, and very few can do this. You must have an Aisling-Level award (By Contest from the Mundanes), along with one Finished Hy-brasyl. Sounds easy, but the Mundanes rarey hold contests and when they do, it's a slim chance to be awarded the highest of recognition.
Aisling of Mythril: Simple enough, sorta. You must have Mentored 200 people (Or one person 200 times, just has to say "Mentored "Student's Name" (200)" on your legend). And you also need 6 Mythril Shields of the same god, doesn't matter which god. These are very common in Shinewood Forest, particularly on the walk to area 22.

Master Sets
Once you have made your final armor and have the legend mark to proove it, you next need a Master set. It is different for each class. These can be collected on floors 55, 56 and 57 of Cthonic Remains. The walk is long, bring atleast 2 or 3 people with Throw to get past the Bridge rooms. And bring a good Rogue and Caster, as there are many creatures swarming all around you in narrow hallways. Remember to talk to Aoife (Lady in Pink) in the Temple of Choosing before going to these places to assemble them.
Warrior: Draco Deathach drops the Broken Jackal's Hilt (Actually the blade) and the Broken Jackal's Sword (Actually the hilt, they got it backwards). Bring these two items to the Dwarf in Abel Dungeon 10-2, and he will put it back together for you.
Priest: Draco Avatar drops the Holy Scroll and Holy Ink. Take these to the Fae in East Woodlands 20-1 and she will write the Priest's Book for you.
Wizard: Ancient drops the Magic Ink and Magic Scroll. Take these to the Fae in East Woodlands 20-1 and she will write the Wizard's Book for you.
Monk: Old One drops the Cauldron and the Gold Sand. Somewhere in the Monk Monastary (I honestly don't know where) these can be made into the Sand Glass.
Rogue: Draco Sgail will drop the Copper File and Smith's Hammer. And, as per usual, the Fae in East Woodlands 20-1 will use these tools to make the Fae's Flute.

Skill/Secret Requirements
Now you have the items ready. Next, the skills. Remember, NO SLEEPHUNTING. It will get you jailed, murdered, etc. Read a book or browse the web while you work on your skills. I drew a few pictures and read Ender's Game.
Warriors require their skills Wind Blade and Wallop to be Mastered (Level 100). This can be very tedious. Head to Medenia and find some chests to work on, but find some against the wall or in a groove so you don't Wind Blade passing by creatures. Keep close watch, as many people will Throw you or try to murder you for no reason whatsoever.
Priests need their spells Ard ioc OR ard noamh aite to be 90/90. Either one will do. It might be good for you to hire a Pure Summoner to Lyliac you, but they can be costly. Shouldn't take terribly long.
Wizards are by far the easiest to Max their spells for. They need ANY two ard spells maxed. If you go Prizard, even ard ioc, ard cradh, etc will work. But ao's don't, so beware. Many people master using an Ard Leasaich spell and Ard puinneag spiorad. They level quite fast and take little mana, I'm told.
Monks get three options to Master. You can learn Brown Dugon and max Wise Touch, or max Clawfist or Beag Nochd. Most go for Clawfist, as it's a very handy spell anyway (Doubles your assail damage).
Rogues require Maiden Trap to be 90/100, so use it whenever you get the chance. They also need their Peek Skill to be 100/100. Have fun with Peek. :P

Health and Mana Requirements
Next, your Health and Mana must meet certain requirements. If you do not meet these, you can Ascend. Ascending is sacraficing experience for Health or Mana. It is calculated by Health*500=EXP Cost for 50 HP, and Mana*500=EXP Cost for 25 MP. To Ascend, you must have made your final armor, and you need a Succubus's Hairs. These drop commonly off of the Succubi, deep in the Mileth Crypt, or you can buy from people for around 100k. With that item, drop it into the Altar in front of the Mileth Inn, and you will be removed from King Bruce's protection. Have someone kill you (Try to avoid inexperienced people, like insight 5 Monks chasing you down the streets). If you are put in "Lag Death," talk to Shamble and step towards the Sigil of Return. If you go to Sgrio's lair with Shamble, walk down towards the rocks. Step into the rocks and you will be put in the middle of them. Float to the northern side of this pile and you will be plunged through a sea of darkness, into the realm of Kadath. There are two temples before you. To your right, Gramail (Mana). And to your left, Deoch (Health). Enter whichever you need and talk to them to begin.
Warrior
Health: 4000
Mana: 3250
Monk
Health: 7400
Mana: 4000
Priest
Health: 3500
Mana: 7500
Wizard
Health: 3750
Mana: 5000
Rogue
Health: 4250
Mana: 2000

Conclusion and Rewards
Now, with your 200,000,000 experience (Which hopefully you have hunted before doing any of this),your stats good, your Master Item made and your Skills/Spells ready, head to the Temple of Choosing. Talk to Aoife, she will give you a long-winded lecture about Mastering, and she will verify each of your requirements. If you have met them all, CONGRATULATIONS! You now have your Master items, and initial Master Skills.
Warrior: Hy-brasyl Escalon and War Mantle.
Priest: Holy Hy-brasyl Gnarl and Sacred Mantle
Wizard: Magus Orb and Enchanted Mantle
Monk: Kalkuri and Sun Bae Nim Mantle/Dress
Rogue: Hy-brasyl Azoth and Shadow Mantle
You may be wondering- Why didn't I get my Helmet? Those are not obtained as soon as you Master. Get a skilled group and go to Grass Fields. Fighting in the Goblin War will reward you with a Legend Mark (Great Soldier of the Goblin War) and your Master Helmet.
You can also learn new skills by talking to the skill Master in Oren (Rogues, Warriors, Monks) or the Faerie Queen in Shinewood Forest 9 (Priests and Wizards). Enjoy.
Becoming a Master also gives you the ability to buy stats. They cost 150 Health for one stat (You can buy them in the Temple of Choosing by talking to Aoife about Master Stats). You cannot, however, buy your stats down below your Master Health requirement. To buy more, simply hunt, ascend, and buy. But remember, the higher your Health, the more experience required for more health. So it's best to stay as close to your Master base HP until your stats are where you want them (Maxed or otherwise). Then when they're how you like, you can start building up your HP. You'll shoot up really quickly, and slow down. There are maxes to stats for different classes.

Class	STR	INT	WIS	CON	DEX
Warrior	215	100	100	180	150
Priest	100	180	215	150	100
Wizard	100	215	180	150	100
Monk	180	150	100	215	100
Rogue	180	100	100	150	215

If you go over one of these stats (Through saved points, Adorations, etc) you can no longer buy stats. You can, however, reallocate stats from any stat over 100 to another. So be careful. After they're all maxed, you can increase them further through Adorations, Redemptions, etc.
Anyway, this guide should have shown you much of how to Master your Path. I learned a lot writing it, and I hope you learned a lot by reading it. Good luck Mastering.

Credit is given to Stadsport for creating this guide.