I have tomato sauce, and I have fresh tomatoes, but I forgot to buy the 28oz can of crushed tomatoes for a sausage/pasta recipe.
Cut a small cross in the ends of your whole tomatoes, dunk them in boiling water for 30 to 40 seconds and remove them. With a paring knife, grasp the edge of the skin where you cut a cross and peel the skin off.
In a large bowl, grasp the tomatoes and crush them with your hands until the size you require is achieved. You now have crushed tomatoes.
Mixing both (the tomato sauce, and some fresh tomatoes) tends to yield a great result. Only fresh tomatoes can be dissappointing: The long cooked, very ripe flavor of processed tomatoes will probably be missing, especially since canned tomatoes tend to be made of tomatoes far more ripe than what is sold as fresh tomatoes.
As mentioned, fresh tomatoes can be peeled using blanching, and then crushed diced or blended; alternatively, cutting them in big chunks, peel and all, and later picking out the peels when everything has cooked off them will work.