I would recommend souring with a know culture rather than with grain. Make the wort you want, boil for a few minutes, cool, add lacto source, maintain around 90 to 100 F for about 7 days (until it is as sour as you want), then add yeast and aerate. You will be drinking it in 6 weeks.
For a lacto source, you could use a commercial brand, but it may be cheaper to use some live probiotics tablets.
Do not use any hops in the wort. Some lacto strains will not work if there is even a 'sniff' of hops present. I like to use dry hops to give a hop presence.
If you use a homo-fermentative lacto, you can boil the wort between souring and adding yeast, at which time you can add hops as you like. If you use a hetero-fermentative strain, boiling will boil off the alcohol created by the lacto.