Hi Kim
I just made some today which was beautiful! I just cooked mine in a saucepan, but I guess you could do it in a slow cooker very easily. No high heat though as it might burn the tomatoes.
Approx 1 kilo fresh tomatoes, diced, chopped, however you like it
2 cloves of garlic, chopped
2 tbs tomato paste
2 tsp sugar
salt and pepper to taste
fresh basil to taste
Olive oil

To cook in the saucepan:
Heat oil, add garlic and tomato paste
Once heated, and fragrant, add the chopped tomatoes
Season with salt, pepper and sugar
Add basil right at the end

I guess in the slow cooker, you'd throw everything in, cook on a low heat and add the basil about 5 mins before serving.

You could add onions, grated carrots, grated zucchini and mushrooms if you like, we just felt like a good old plain tomato sauce today!