When I scratchboard I rely heavily on the image I am working off of; the better the photograph, the better the scratchboard. There are some simple tricks to take a strong photo for a scratchboard portrait.
First tip: Take lots of pictures! You can then pick your favorite (if your taking a picture of people, and want the scratchboard to be a surprise, just pretend your going through a photography phase)
Second tip: Get a high quality photo! The higher the resolution, the more detail I can put into the portrait! The devil is in the detail after all.
Composition
Rule of thirds: If you’ve ever taken a photography class you’ve probably heard of the rule of thirds. This is used to create an interesting composition for your portrait. First, identify about 4 points of interest in your image, and then frame the image so that these points fall on the intersections, or lines of the grid. This will make 
your photo more balanced and will enable a viewer of the image to interact with it more naturally. Studies have shown that when viewing images that people’s eyes usually go to one of the intersection points most naturally rather than the center of the shot – using the rule of thirds works with this natural way of viewing an image rather than working against it.
Perspective: This section is especially important for photographing pets. Try to take the photo from eye level, and yes for pets that could mean having to get down on the ground. Photos of pets shot from above can make your pet look stubby, and you will frequently lose the neck in the picture. Taking a photo from below eye level can give your pet or person an aura of authority. Just look how politician are normally photographed: 
Take a few pictures close up to the face as well, so that it fills a large portion of the frame. Try taking some photos where the subject is not directly facing the camera as well.
Lighting
Pets: The preferred lighting for photographing your pet is natural light, so the best place to photograph your pet is outside! If your pet is indoors only, take the photo near a window. DO NOT USE FLASH, and avoid direct sunlight, as these will alter the true color of your animal. Flash will also create glare on fur and eyes, taking away its natural appearance. A slightly overcast day can make the best lighting for photographing your pet.
People: As with pets, the best lighting for taking a portrait is natural light. Get your subject outside, or near a window for the best lighting effect. The best scratchboard portraits of people are based off of photos with intense lighting and contrast. Found this image… now I really want to scratch it! 