Partial Mashing with a Cooler / Chilly Bin / Esky

Depending where you live in the world, it'll either be called a cooler, a chilly bin, an Esky or other. Getting hold of one is easy. You can buy one new from a range of stores, or you can go the budget way and look for one second hand.

A 7.6 litre (2 gallon) cooler, plus a large nylon grain bag is all you need to move from extract brewing to partial mashing (or mini-mashing). It requires no modifications to the Esky.

The process is this:
Mash 1.8kg (4 lbs) of grain in a 7.6 litre (2 gallon) cooler.
You use batch sparging to collect 11 litres (3 gallons) of wort
You then boil the wort, add your hops as you normally would with extract brewing. This shoudl reduce the wort from 11 litres down to 9.5 litres with evaporation.
At, or near the end of the boil, you add some additional malt extract (dry or liquid) to make the wort up to the desired level of fermentables.
After cooling the wort, dilute it with boiled and cooled water to make up your total amount (usually 19 litres / 5 gallons).

For more details on the process, check out Countertop Partial Mashing on BYO.com