Even with multigrade papers and split filter printing, many images will require some selective exposure. Either a dark area needs to be held back some - dodging, or a highlight area needs extra exposure - burning.
How and when you burn/dodge depends on several factors. The first is the negative itself. Is it particularly flat or contrasty? Is it under- or overexposed? Be realistic about your evaluation. A negative that is too far from good, or normal, may produce an image, but may also be source of frustration in the darkroom. Darkroom techniques should not be seen as salvage operations, but rather as refinements.
A typical use of burn/dodge is with landscape. Since panchromatic black and white film is more sensitive to blue, often the sky in landscapes is overexposed. To get detail, or even tone, the sky area is given more exposure than other highlights in the image. (Keep in mind that is you do a lot of landscapes, it's worth investing in a yellow, orange, or red filter to help alleviate this problem).
When printing by any method, with graded or multigrade paper we start with a base exposure, tested to get the dark areas correct. Those dark areas on the print are produced by the thinner areas on the negative. This base exposure may well not be enough to get through the highlight areas, the densest part of the negative. Adding exposure beyond the base will only fill in the shadow areas, losing any detail we might have. So that base exposure is gotten to by testing, starting with the important dark (shadow) areas.
Once the base exposure is found, make a test print of the entire image. This test print will tell you where further adjustments need to be made. Examine this print carefully, under good light. First look at the important shadow areas. Do they hold the amount of detail you want? The tendency for beginners is to print too dark. Now look at the important highlights. Do they hold detail? Are there areas of less important highlights that are completely washed out? This is where selective exposure comes into play.
The procedure is pretty straightforward. Dodging, or holding back exposure means the areas affected are masked during exposure, for a tested amount of time. Conversely, burning gives an area more exposure, while masking the rest of the image. The mask needs to be moving, in order to feather or blend the two different areas of exposure. The procedure does take a little practice.
If the majority of the print looks fine with the base exposure, but there is a shadow area that is too dark, then that area should be dodged, or the exposure in that area should be held back (less) than the base exposure. Start by retesting that specific area, using finer increments than the base exposure. Try cutting the time increment in half. This is in order to get a finer, more exact test. If the original, base exposure was rather short, under ten seconds, you may want to stop the enlarger down, instead of altering the time increment, or in addition to. Remember, one stop smaller on the lens will double the amount of exposure you need.
Retest the too-dark dark area. This will give you a shorter exposure than the base, and tell you how much less exposure that area needs. Proceed with another test print. Start with the new, shorter exposure over the entire image. Then, using your hand, a card, or a dodging tool, cover the area you need to dodge. Add the remaining exposure, keeping the dark area covered, simultaneously moving the mask, either up and down or side to side, in order to feather the edges of the two different exposures.
Burning, adding exposure, to a highlight is more common than dodging. As above, the most typical need in in a sky area. Many times the sky will be completely white, which is particularly problematic when the edge of the image is undefined. Test that highlight area, giving it the total base exposure first, then a geometric test beyond. Do the geometric, as an incremental (arithmetic) test may not get you far enough. Burning in skies can take a surprising amount of exposure, compared to the base exposure.
Once the test is done, and the additional highlight exposure is determined, first expose the entire print for the exposure determined for the shadows (the base exposure). Then, set the timer for a few seconds more than the additional amount of exposure needed in the highlights. Using a mask (your hands, some board, shaped or not) cover the lens (blocking the light). Start the timer, and move the mask to cover what you need to cover, exposing the rest of the print for the time needed. The additional time is to allow for the placement of the mask. Like dodging, keep the mask oving in order to blend the edge of the additional exposure.
Split Filter Considerations
When printing with the split filter method, the selective exposure is done during the exposure for the corresponding filter. When dealing with darks, and selective exposure is done with the #5 filter. Likewise, when dealing with highlights, the selective exposure is done with the #00 filter.