EAC grading is always a "hot-topic," so I am going to take a crack at putting my perspective on it here!
Basically, EAC grading is an approach that is more conservative in general than market-grading or slab-grading. For example, an EAC member will not grade a coin in an AU50 holder as an AU if the coin has NO LUSTER! If the coin has AU wear but no luster, an EAC member would grade it as EF45 at best. And if there is a MS-graded coin in a holder that has mint state luster but also has a light rub from circulation, the EAC grade would be AU at best!
The basic process of EAC-grading starts with the ANA Grading Guide. If you don't have one, spend a few bucks and pick one up. But keep in mind, that is just a STARTING point - the real trick to becoming an expert grader is to look at as many coins as possible! If you are not able to look at as many coins in person as you would like, then another excellent resource are auction catalogs, particularly those that incorporate EAC-grading. As of today, the best source for those are the Goldberg catalogs, since the grading is done by long-time EAC grading expert Bob Grellman. Many of the Goldberg catalogs are posted online, so you can spend lots of time browsing the photos of early coppers and comparing those photos to Bob Grellman's description of the coins. One of the things that I really struggled with starting off were the grades from VF20 to EF40. It's pretty easy to peg a VF20 or an EF40, but what about the three grades in between (25, 30, 35)? The short answer is that you just have to look at enough coins to be able to make that determination.
Once you get a good feel for the starting grade of the coin, it's time to make some deductions for any problems, and herein lies the core of the problem with EAC-grading. Let's say that we have a 1798 large cent that is a VF20 or better according to the ANA grading guide. The coin is original with nice color, but there are three tiny rim bumps that are visible only on the reverse. So how much do we knock off the grade for those 3 tiny rim bumps? That answer is going to be DIFFERENT for darn near everybody! For somebody putting the coin into an album where the reverse won't even be visible, they might not knock off ANY points from the grade (VF20+ net VF20). But for the person whose main focus for their collection is SYMMETRY, then those tiny rim nicks might be a huge problem and a huge deduction (VF20+ net F12). I have two points to make on this: FIRST, man-made problems are typically more of a deduction than mint-made defects. For example, a coin that has been cleaned (man-made defect) will typically be graded harsher than a coin that was struck on a planchet with flaws (mint-made defect). Furthermore, deliberate damage is worse than "regular damage" that might have been incurred simply from circulation. For example, a coin that has been whizzed (mechanically polished to make the surfaces smoother) would be treated less kindly than one that has several tiny nicks from being in somebody's pocket. SECOND, some people use what I will call the "would I rather" approach to net grading, and it goes like this: would I rather have this VF20 with slight problems, or that F12 with no problems"? If you would rather have the VF20 with problems, then the "net grade" is F12 or F15, since you like that coin BETTER than the F12. If you would rather have the F12 without problems, then the net grade on the VF20 would be F12 or LOWER, since you think the F12 is a better coin.
So as you can see, if you have ten different people assign an EAC grade to a coin, you are probably going to have AT LEAST three different net grades, because different people have different opinions of the various problems that these coins are afflicted with, and they are going to punish the coin according to how much that particular problem bothers them.
Now let's throw another wrinkle into the equation: die states and various variety peculiarities! Typically, coins struck from broken dies are going to be unevenly struck, either in isolated parts of the coin, or else over the entire coin. Since the dies are broken, some of the pressure from the strike is directed into the broken parts of the die instead of onto the surfaces of the coin, and this causes not only cracks and cuds, but also poorly struck areas. So how does that affect the grade? The short answer is that it really shouldn't affect the grade, but that it probably could affect the value. I think most people are like me in that they would actually PREFER an early copper with a dramatic, devastating crack or rim break, but there are also those that prefer fully-struck, early die state coins with even strikes on both sides and all details fully-developed. So a coin that is really a VF20+ by details but exhibits some areas of weakness due to the die state might be assigned two different EAC grades by those different people.
Furthermore, many varieties are just "struck that way." A great example is the 1804 large cent: they are all struck such that the top left corner of the reverse is always weak, and as the dies broke and the two cuds developed, that area gets even weaker still. Somebody that is not an expert on 1804 large cents might look at that coin and think it was unevenly worn in that area, when in fact, it was not struck up in that area from the very start. In cases like these, you just have to assign a grade based on the REST of the coin, knowing that the top left corner of that 1804 is ALWAYS going to look that way. In addition to making you a better grader, learning the different characteristics of the varieties can also be a huge plus with respect to authenticity. For example, there was recently a very convincing 1804 large cent being offered on ebay that I knew right away was a fake because the left reverse was much stronger than the right. The real coins just don't come that way! Educate yourself!
Okay, back to the final portion of EAC grading, and this one is a little bit simpler. Once you have assigned a starting grade, made a calculation of the deductions based on the problems to arrive at a net grade, the final step is to assign a CONDITION to the coin. The condition refers to the color and overall eye appeal of the coin, and there are 5 possible choices: Choice, Average Plus, Average, Average Minus, and Scudzy. With a few exceptions, this part of the grade is usually less controversial. For example, with the 1798 illustrated above with 3 tiny reverse rim bumps, MOST people would call that coin AVERAGE because it is totally original with nice surface and color. Depending on how incredible the color and surfaces are, some might even call it PLUS, and on the flipside, if the color and surfaces are sort of lackluster or below average, then some might call it MINUS. But typically there is more agreement in this area than the rest, and once again, the more coins you look at, the easier it will be for you to know what an "average" coin should look like.
So back to our example, I think my EAC grade for our illustration coin would be VF20+ net F15 Average. The rim bumps are tiny, the coin is original, the color is a pretty light tan, and the surfaces are clean for the grade. So, what's it worth? Now that is a whole 'nother can of worms!