I think 1 to 1.5 lb of fruit per gallon is usually good. I freeze the fruit first, which makes it easier to smush and should kill most critters. Or you can heat the fruit with some water to 165 (pasteurization temp) but I prefer not to risk setting the pectins. Or you can mush the fruit at room temp and add some crushed campden tablets (potassium sorbate, I think. Or sulfate. Or something.) but I prefer not risking killing the yeastie beasties.
Then add to secondary. Using a mesh bag is nice so when you're done you can just lift the bag out with all the pulp. This works best with a bucket style fermenter obviously, but I've actually added fruit to a glass carboy which is possible though certainly a PITA to clean. Hint: put fruit in first, then rack on top of it. Then if you run out of space, you can just bottle the remainder or put it in a second carboy or jug, rather than not being able to add as much fruit as you want. Make sure to leave plenty of headspace because I have found that adding the fruit usually kicks off an exuberant fermentation.