I did the Guinness thing once, many years ago, and have done a few sour beers since. In either case, sour mashes/worts do not work well unless you can hold the temp at ~35C for 2 or so days - lacto doesn't grow well below those temps. I've come up with a somewhat optimized process:
1) Cool your wort/mash to 35C and throw in a bit of uncrushed
malt. For a sour-mash you need a handful; for the litre or so you'd be souring for a stout you'll only need a few kernels (and can probably use crushed malt).
2) Cover the wort/mash with saran wrap. Some people will coat with CO2 instead. In my experience saran wrap works better. What you are doing is creating an oxygen barrier - laco is an anerobe, so O2 has to be kept off.
3) Hold at 35C to sour; it is best to taste it every 12 hours or so, to see where the sour level is. You wont want to taste it - it smells like dirty feet dipped in vomit* - but tasting is all you can do to see where it is at. For most sour mashes/worts, I've found 48 hours is usually about right. I hold at this temp using an aquarium heater inside of my mash tun; an oven or smoker can also work - ideally outside of the house.
*Don't worry, this disappears as soon as you boil.
4) Once soured, skim off of the pellicle (snot/wax-like layer of lacto that will form). If souring a mash, you can now sparge it like you would sprage a normal mash.
5) Boil it. In your case, 15min should be sufficient.
6) Apologize to SWMBO'd - your whole neighbourhood now smells like dirty feet, and she will (likely) be pissed. I find flowers work well - and help cover up the smell
DO NOT do the souring, or any transfers of unboiled sour materials, using any of your beer-handling plastics that encounter post-boil beer. The bacteria will find every little crevice and will then contaminate every brew you make afterwards. In your case you can probably sour & boil in a small pot, so its a non-issue. For anyone wanting to do sour beers, you need to buy a second set of siphons, air locks, and other plastics for these sour beer.