After I determine the time to dodge/burn, I usually set my timer into intervals of, say, three seconds. So, if I like how the print looks at 15 seconds@ f/11, and decide that at the same f/stop I want to burn in an area for an extra two seconds, I would first burn in the corner for two seconds, then carry on my exposure as per the usual manner. For burning, if the correct exposure is the same as the example i used above, but one area is two seconds too dark, I would set the timer to two seconds, do my burn exposure, and then do the whole print for 13 seconds. I'm not the greatest at super creative dodging and burning, and I usually do it only when the print has a very visible flaw or I want it to look perfect, and I am being very general with my procedure, but it works for me.