Failures and how to set them

I've read the manual, but am still trying to figure out how to set failures. I would like failures to be rare and random, but when I check the random box, go back into the sim, I've got a couple of failures already. Clicking on the random box brings up a time and type in the counter/type box as well. Should I first set the counter/type, then place a random check in the box?Thanks,