Original: https://learn.kegerator.com/brewing-sour-beer/

Mash at 152° Fahrenheit for 30 minutes. Starch conversion is not as important here, as the wild bugs will chew up the complex starches in secondary.
Mash out at 170°, then bring to a boil. As it starts boiling, add your cheesy, nasty old hops to the kettle.
Boil them for 90 minutes, then chill the beer and rack it to primary.
Pitch the dry yeast and seal it up for 14 days.
Once 14 days have passed, you can then pitch the lambic directly into primary and perform the secondary in the same vessel, or you can rack to a clean fermenter for secondary if you are more comfortable with it.
Let sit for 6 months, then take a gravity reading.
You can bottle at this point with some corn sugar or you can continue to let it age for as long as you like.
Variation:
After primary fermentation is complete, rack the beer onto at least 5 pounds of fruit in secondary. Make sure the fruit has not been treated with a bunch of chemicals at the grocery store. Buy from a local orchard if possible, or, if you can, get some IQF (Instant Quick Frozen) fruit. IT is commonly sold commercially, but is hard to get for home use.

You can do as much as ten pounds of some fruits if you like. Stone fruits like cherries and peaches are excellent options, but peel the peaches if you use them. Let it ferment for the full 6 months, and then taste it and take a gravity reading. If you’re happy, bottle it. If you want to give it more time, let it ride.