I recommend crushed tomatoes for the best results but there may be a need to use a different type of canned tomato. Here's my recommendation for both whole tomatoes and diced tomatoes. To use whole tomatoes, add the contents of the can, liquid and tomatoes to the pot. Use a wooden spoon to crush the tomatoes and break them apart. If you can't get the right consistency you can also use a potato masher, just be sure to not over mash the tomatoes to the point where there are no chunks. If diced tomatoes are all you have, you will need to take a little extra step. I find that diced tomatoes won't crush in the pan with a spoon like the whole tomatoes while the marinara sauce cooks. Put about 1/2 the contents of the can into a blender or food processor and pulse a few times to chop the tomatoes. Don't blend to a puree just chop them a tiny bit. Then put everything in the pan and continue cooking.