Any pale malt will work. I used pilsner because that's what I had.
Do not boil. Strain, and cool to 120 degrees.
Put 2 to 3 ounces of whole pale malt in a gallon jug and transfer the wort over it. Top up all the way with boiled (deoxygenated) water and cover with plastic wrap and a rubber band. Don't leave more than 1/2 inch of headspace. Hold at 118 degrees for at least 3 days to sour, then it can be stored at room temperature.
Use to acidify the mash of future beers (it should be about 2% lactic acid) and top the jug back up with fresh unhopped wort to keep it going.