Very simple, actually. Once I run off my boil volume, I add more sparge water and run off about a gallon of wort. The lower gravity the beer, the more you need to run off and the less return you will get out of this process. The reason is that each time you take new runnings the sugar content is going to be less and less, so by the time you are taking runnings beyond your boil volume it is a very diluted wort. The lower gravity your beer is going to be, the less sugar you start with so the more diluted this last running will be. At any rate, I take a gallon or so out and boil it down to 1.030. You will need to use your hydrometer and either cool the sample or adjust for the temperature. I just adjust for temperature until I get somewhere in the 1.030-1.040 range.
Once I reach the proper gravity range, I cool the wort and pour it into tupperware containers and toss them into the freezer. It helps to label what beer they came from, but it is not absolutely necessary. I find those reusable containers from pre-sliced deli meats to be great for this purpose. You can freeze about .5 liters in them. They are easily stacked and fit in small spaces, so it is easy to work them into the freezer. I find that these containers handle the swelling from freezing and do not leak from defrosting. I previously used this process by sealing the wort in freezer bags but those bags develop small holes from the expanding and contracting during the defrost cycling in the freezer.
Whenever you need to use it, give it a quick boil and cool it down. I find with tupperware the wort can sometimes be hard to get out so I minute in the microwave will dethaw enough to get it out.
So that is the process. Really simple. You may want to boil it down to an even higher gravity and add water later. At some point I will probably adopt that process instead as I run out of freezer room. That is something to think about.
1. The most obvious use is for making starters. Although some people do can wort for this use, I lack canning equipment and the fear of botulism scares me away. Plus, freezing is much easier. If I need to make a starter I can just pull out a container of wort and have a starter within about the same time as using DME, but without the added cost of buying DME.
2. Making graff. Graff is fermented cider made with some portion unhopped or lightly hopped beer. Woodchuck cider is actually graff; that is why it is so much sweeter than other cider products on the market. Graff is not only tasty, it is also much easier to ferment than straight cider because the malt portion adds minerals yeasts use during fermentation that apple juice lacks. The malt portion contains unfermentable sugars so it will add sweetness to the apple juice, which will otherwise ferment out and become very dry. Generally graff recipes contain 10-25% wort.
The recipe is super easy. For one gallon follow this process (for more gallons just multiply by the number of desired gallons) Buy one half gallon bottle of apple juice. Store brand is fine, although you may prefer using a juice that is a specific kind of apple or a blend of apples. The only restriction is that you need to buy juice that does not have any preservatives in it. You will often find apple juice contains Ascorbic acid, which is vitamin C. That is ok, but anything potassium or sodium based should be avoided. I know store brand juice is typically juice concentrate, water and ascorbic acid. Preservatives will severly slow or stop fermentation (hence the reason why they are added). Also buy one can of frozen apple juice. You can also use store brand. Similarly, watch out for preservatives. Because both of these products should be pasteurized, there is no need to boil or apply heat. Just add the juice and concentrate to the fermenter. If by chance you have bought unpasteurized juice, you will need to at least bring it up to 160F to pasteurize it and kill off any wild yeast (unless you want the wild yeast flavors).
At this point you have super strong apple juice. Boil up some frozen wort, cool and add to the fermenter. For one gallon I find .5 liters is enough to add some sweetness and flavor to the graff without taking away from the apple flavor. Top up with water and ferment with whatever your preferred yeast. I often use 1338 because it is very neutral, but others use wine yeast, champagne yeast, Nottingham, US-05, US-04 and various English yeast strains.
Unlike regular cider, which will ferment completely dry if you do not stop fermentation, you can let graff ferment out completely and still bottle carb with priming sugar without worrying about bottle bombs or having an unpalatably dry product, thanks to the unfermentable sugars from the wort. If you do want it to be sweeter, you will need to stop fermentation early, in which case you cannot safely bottle carbonate. If you do let it ferment out, you will probably want to let it bulk condition for 4-5 weeks. Otherwise, you will get an off smell from the graff often referred to as "rhino fart".
Do not feel like you can only use neutral wort for graff. Feel free to experiment. Right now I have a graff fermenting that is using some extra wort from a Belgian quad I brewed, so the graff will have small amounts of flavor coming from aromatic malt, special B, crystal 120 and biscuit malt. I almost fermented it with a Belgian yeast, but logistics called for using 1338.
3. Yeast harvesting. You can also use the wort for bottle harvesting and catching wild yeasts. Having a little wort on hand makes it easy to bottle harvest without worrying about whether you have DME on hand to make a quick starter. You can also use your extra wort as a medium for catching wild yeast.
I feel like there are some more uses that aren't coming to mind. At any rate, this is a pretty simple process that pays off. I haven't bought DME in a long time because of it. I can run off extra wort with sparge water while waiting for the main brew to start boiling and take care of boiling the extra wort during the first half of the main boil.