I've tried making sour-mashed p-guinness before and also making it by adding sour beer to the boil, and the latter works much better. The sour mash leaves somewhat of a raunchy flavor to the brew, whereas simply adding to the boil a bottle or two of beer from a previous batch that has acquired a lactic infection produces a crisper, cleaner sourness.
How much to add is always subjective. To get you in the ballpark - if you can smell the soured beer a mile away, add 1-12 oz bottles in 5 gal. If you can actually drink it w/o throwing up, add 3 or 4 bottles. If you got lucky and the soured beer actually tastes funky-lambic-like sour, add a gallon.
The boil will kill anything in the sour beer, but make sure you clean the bottle up and anything outside the kettle that you may have splashed with the sour beer well before the boil ends.
The whole idea is to keep the protein in the beer, so you start with Pilsner malt & don't do a protein rest. Mash using you favorite technique, but keep it short - 1hr or so. Sparge w 170 F water (acidified). Do not recirculate excessively. The short mash and the pilsner malt will help avoid a stuck runoff. Bring the wort to a boil as quickly as posssible. Normally I boil 30 min to coagulate the protein before I add hops, but i in this case, add the hops right at the start of the boil, or even before. Use Goldings. Add the soured beer - preferably soured from a lactic infection. Boil 1 hour, or 45 min if you used hop pellets instead of plugs. Cool & pitch Wyeast 1084 Irish ale yeast starter. SG should be 1.045-1.050 or so, unless you get spectacular extraction rates (I don't). Ferment 60-65F.
Now if you bottle, use 3-4 oz corn sugar and let condiiton. If you keg, you've got an added element in how you imitate guinness: Chill the beer to 50F, & turn the pressure up to 10-15 PSI & Serve. Do not agitate the keg. The beer will have a head, but very little carbonation in the beer itself, just like guinness.