Before I start my guide i think we should know who garen really is. I cpied this from the LOL site itself so hopefully I don't get in trouble.

STORY:Throughout Valoran, the resolve of Demacia's military is alternately celebrated or despised, but always respected. Their ''zero tolerance'' moral code is strictly upheld by civilians and soldiers alike. In combat, this means Demacian troops may not make excuses, flee, or surrender under any circumstance. These principles are espoused to their forces by unrivaled demagogues who lead by example. Garen, the valiant warrior who bears the title ''the Might of Demacia'', is the paradigm to which these leaders are compared. Thousands of great heroes have risen and fallen on the bloody battlefields between Demacia and its preeminent rival, Noxus. It was beneath their mighty banners of war that Garen first met steel with Katarina, the Blade Mistress. The infantrymen who beheld this event (and survived) commented that it seemed as though the two were locked in a mortal waltz against a symphony of clashing blades.

Garen, the pride of the Demacian military and leader of the Dauntless Vanguard, returned from this battle breathless for the first time in his career, though some speculate that this was due to reasons other than exhaustion. The plausibility of these rumors was bolstered when, in every instance thereafter, Garen seized the opportunity to encounter the Blade Mistress again. A paragon of Demacian ethic, Garen never entertained such allegations, for he knew others couldn't understand. Even simply the pursuit of a worthy opponent on the battlefield is, to a true warrior, the reason to rise each morning. The promise of one, particularly so beautifully and diametrically opposed, is the validation of his existence.

''The most effective way to kill an opponent is to slice through the man next to him.'' - Garen, on front line strategy.

Well, now that you know who he is, lets begin!

RUNES:
Basically the marks increase his critical chance. The seals increase his attack speed. The glyphs reduce cooldown time. The quints allow gold per second, increased experience, and movement speed.

MASTERIES:
High critical is essential in this build. Max deadliness then Cripple. Next set get some of Sorcery and aclarity and get extra later on. Sunder gives the armor penetration and we get some Brute force to imcrease our max damage. Lethality is a must. Improved rally is only beneficial for the extra time so using it near a spellcaster is a nice way of saying, "lets beat them down." Havoc is for the max damage.
Max good hands then get one point in preserverance. last three points are spent on preserverance to level faster in battles.

SPELLS:
Exhaust: slow down opponents to finish them off.
Rally: extra damage with our attacks

If your to low to have rally, get ignite. If your to low to get ignit get smite and its mastery point to help cash flow.

The sequence is really the final six: the last six items you should have when you win. If you get four or five, its all good. three or less means that either this guide or Garen is just not your style.

This is the detailed version:
Start off with getting Long sword (1/6, 0 final six). its needed to build two of the build's "Final six." Fight until you can but Boots of speed (2/6,0 final six). Purchase Berserker's greaves when you have enough(1/6, 1 final 6). Purchase cloth armor(3/6, two of final six) then purcahse Madred's razors(2/6, 1 of final six). Purcahse a long sword (3/6, 1 of final six) and then purchase phage (3/6, 1 of final six). Purchase a dagger (4/6, 1 of final six) then upgrade to zeal (4/6, 1 of final six). Upgrade zeal tp Phantom dancer (4/6, 2 of final six) then upgrade phage to Frozen Mallet (4/6, 3 of final six). go creep hunting and purchase Atma's impaler (5/6, 4 of final six). Advance Madres's razors to Madred's bloodrazor (5/6, 5 of final six) then Purchase Infinity edge(6/6, 6 of final six).

There are other ways to do this but this is a good way.

Decisive strike is an effective way to weaken champions early on when you have low attack speed an combined with its silence effect can help you fight some of the more annoying champions.

Courage is only good for improving armor and magic resist or jungling. Max this last.

Judgement is a great jungling skill. it can kill a group of wolves or wraiths in seconds.

Demacian justice is the "KILL SKILL!" use when the enemy champion is at 20-50% of their max health.

Use a mix of Decisive strike and Judgement when smashing turrets. Decisive strike works on the turret and judgement is how you can kill enemy minions nearby.

Champ killing works in 2 paths: Demacian justice available or it not available. If its available, use decisive strike then courage. if they are fleeing use judgement then finsh with Demacian justice. If its on cooldwon, your chance to kill a champ at full health is harder to do but if you can get a few more decisive strikes in the mix, you may kill it.

JUNGLING:
start with judgement. finish with decisive strike n anything that lives. use courage when getting runes or fighting dragons.

OUTNUMBERED:
1v2: if ones weak use demacian justice on that champion then use decisive stike on one then run. Activate courage then use judgement to stop slow effects and dmg them when they are to close.
1v3: Courage. decisive stike but don't attack. Run away and use judgement.
2v3: same as 1v2 but longer.
2v4: yeah. this only happens on one map and very rarely. Focus all damage on one target at a time, using demacian justice on the weakest. Judgement works for weakining all at once. if ally dies, run.
3v4: same as 1v2 but way longer.

1. 77.99% critical
2. 270.07% critical damage
3. 235 base damage
4. 126% attack speed
5. 447 movement speed

With these 5 here, you should be able to sneak around enemy lines and blast their nexus open like a kid with a presnt.

1. Low health
2. Low defenses
3. Very selective set of items.

If you don't like 2 of the 3 here, don't try thr build.

Anyway, thanks for looking at this everyone. My first guide and not the last. Ask JASLegend, me, if you see me in game and say what you think.