The goal is to provide sugar for yeast to ferment so they can make alcohol. To do that using grain, we need to mash. The two main challenges in mashing are (1) to release starch from the grain, AND (2) convert that starch into sugar.
Releasing the starch requires water and heat. Some grains (like crushed barley and wheat) release their starch at low temps like 145F, and some grains like corn require high temps, like 180F . Crushing barley, or cracking corn helps the water get into the grain and dissolve the starch. Ok, that's pretty simple, eh?
Don't forget about item 2 up there . We need to convert starch to sugar , and that requires enzymes. Where do ya get enzymes? ….well ,you can go to the brew shop and buy 'em in a bag ….but they're present naturally in malted grain . Thats why brewers use malted grain and not raw (unmalted) grain for making beer.
A word about enzymes though. There are a couple different types, and they work and denature at different temps. Higher mashing temps like 156F will leave you with a beer that has more unfermentable sugars than if you mash at 146F. Since distillers want all the alcohol they can get out of the grain, it makes sense to mash at a low temp (145-150). As a rule enzymes don't like heat above 160F.
So knowing what we just learned from those last 3 paragraphs, we could easily take a mess of crushed malted barley, and mix it with 150F water at a ratio of 3 pounds of grain/gallon of water and make some beer. Mash that for 60 minutes in a beverage or water cooler and it's pretty likely that all the starch would be dissolved out of the grain particles, and converted into sugar by the enzymes. If you tasted it after the mashing session, it would taste sweet. If you cooled that down to 80 F and pitched some yeast, it would certainly take off and ferment. You could do the same with crushed malted wheat , or crushed malted rye. This is called a single step infusion mash.
So that was pretty simple, but what if we wanna use cracked corn in the recipe? ...Cracked corn has a lot of starch in it , but if you try and do a single step infusion on a mess of corn at 150F, nothing's gonna happen because (1) 150F isn't high enough to dissolve and gelatinize the starch in the corn, and (2) there's no enzymes in cracked corn, so even if a small amount of starch was released from the corn, there'd be no conversion of the starch to sugar. The solution is simple....cook the corn at a higher temp FIRST, and then cool it down to the mashing temp an add your enzymes, or malted grain.
Here's table of all the usual grains and the temps that the starch is released and gels.
The procedure in the Carolina Bourbon thread is pretty easy to follow for a corn and barley mix: Carolina Bourbon
Lets talk a little about flaked grain for a minute because flaked is already cooked, that means it's pre-gelatinized. The most common flaked grain you'll see is flaked corn. It's more expensive than cracked corn, but it's easier to brew with because you can use it in a single step infusion mash. It has no enzymes in it (remember?, it was cooked) so you need to add commercial enzymes or a healthy dose of malted barley in the mash to get the conversion of starch to sugar done. As you can imagine, usuing flaked corn in a bourbon recipe can be a real time-saver since you can just add the flakes into the mash just like you do with the malted barley.
You might be wondering how you figure out how much malted grain you add to the recipe to get the conversion done … In the world of malt, there's something called diastic power. It's basically a measure of how much enzyme the grain has, and it's ability to convert not only it's starch but other starches (like corn starch) as well. Modern 2-row base malt that you would use in a beer has a DP (or L (Litner)) of approximately 150. You typically need a minimum of 30 litner per lb of grain to get a good moderately fast conversion in your mash, assuming that you do everything right. So lets do a little math ...150L divided by 30 = 5 , so you could add 4 pounds of a NON-MALTED grain (corn?) to 1 pound of this 2-row barley and still have enough enzyme power for proper conversion. Got it?
A word of caution on buying malted grain, some malts you see in the brewing shops do not have any enzymes ...caramel malt, biscuit malt, honey malt, chocolate malt have all been cooked/roasted and the enzymes have been killed. 2-row base malt and 6-row base malt, or distillers malt are what you need for a mash with other raw, or non malted grains.
Whole books have been written on this, and there's a ton of information on the site here, as well as brewing sites on the web, but I hope this gives you some basic knowledge that you can put to use.