Boots are covered in optional item section.

The basic outline of this build is for you to be a meat package that absorbs damage like no other, and also kills a lot of people. The reason i build my items the way i do is that once you get warmogs you can lane insanely long and if laning phase is over you will be able to start stacking its abilities so you have even more health regeneration. With this build you will be a tank destroyer and a squishy annihilator! Your W is like a portable sunfire cape, use it! But, use it wisely. I see a lot of people using it and dying because of the constant health drain! Your basic attack should be fine, and if you have a good dps champ that builds sark's fevor, then your going to be even better! The reason i build thornmail a lot is because almost every game has high DPS with low mag resist. And since your the main target among groups you kind of get it bad from the lil squishies that fling swords, lamp posts, or pesky little arrows all of the time.

Pros:

Great harrasing abilities early game
Great laning survivability
Awesome off-tank
Can push lanes like a mofo
Goes where he pleases
Low CD

Cons:

Ignite shuts you down
Usually gets assists
Slow attack speed (Not needed in my honest opinion)

Early Game: Just harass the **** out of enimies, take bot or top lanes. You can often times get a kill or two by lv 6! I don't gank others on mundo just because the other lanes arent as easy to pick off after a few cleavers have been spammed into their pesky little boddies.

Middle Game: You should have warmogs now or damn well near close to warmogs. Start by helping where its needed, if people are pushing bottom quite hard you use teleport and get in there and let them axes fly! The more you use mundo the more you will spam his abilities and get good at timing, so don't get mad if your new to mundo and your just not pushing people down fast enough.

Late Game: You will have Visage, 2 Warmogs, Force of Nature and be working on your thornmail or option items by now. Optional items covered in different area!And you will be a force to be wreckon with. Stick with the team stay in the middle or lead them, if there is another tank hes the main tank not you, your job is to push and find the squishiest sons of *****es out there and **** their **** up. Use your W as it will be helping in demolishing their **** up. Just spam cleavers and they cant escape! People will probably use CC on you so just cleanse it off if its at all risky or multiple CC in a few seconds. You can push and lead your team to victory with over 5k HP, doing 400-500 damage/hit to the enimies main tanks, and 200-300 to squishies with your cleaver. Dont use ult untill about 1,000-2,000 HP! And, when you do use Ult, GTFO! your VERY vulnurable now, all somebody has to do to kill you is to use exhaust and ignite, instant death.

First off is the boots, as you noticed I don't specify any boots mainly in the game. That is because every game is different, get the boots you think you are going to need. The new boots with 15% CDR are a good option for more cleaver spamming. However if you don't take teleport as a summoner spell you are going to need boots of mobility. You need to be where the action is at all times after mid game. If your not, your team will usually fail untill they are feed a little ;).

I don't know how to insert any of those fancy pictures or little sprites so forgive me.

Your optional items are these (depending on scenarios):
High ammount of mages: Quicksilver! Its better than banshee's veil, trust me!
High ammount of ranged users or mages that flee a lot: Frozen Mallet
Can't seem to escape all of these ****ing invisible champs and i die a lot: Guardian Angle (or use buddy system ingame!)

The rest of the many items that are obviousy for a scenario are quite optional aswell, for example... You have been assassinated by evelynn when leaving the base and are on low Health, obviously oracles is a good counter. lol.

If you are getting a ****load of kills you may want to get Leviathan! At 20 stacks YOU BECOME 15%+4%=19% RESISTANT TO DAMAGE!! WHALO! 4% came from masteries!