I've included sour mashes in 2 of my beers: a Saison and a Wit. On both, I was looking for tartness, not an all out sour character.
My technique is to take some of the runnings, cool them to ~90F, toss a handful of crushed grain in, and let it sit for a few days. Then strain out the grain and boil the liquid for 15 minutes. In both cases, the end product smelled godawful, and the taste was incredibly sour but in a sort of clean, mostly unoffensive way. I think you need to approach 5-10% of your total wort being soured for it to have a noticeable effect.