It's easy too. You plonk 5-6 tomatoes in a pot of boiling water for a minute, then put them in an ice bath. This should make the tomato skins easy to peel off and you can then cube them, minus the seeds. Also chop a small onion and peel & mince a couple of garlic cloves. Heat a tbsp of olive oil, add the onions and cook until translucent. Add the minced garlic, wait a few seconds then add the tomatoes and salt. Simmer for 10-15 minutes, mashing it a little as you go but leave your sauce chunky.
Chop a handful of basil leaves and add to the sauce towards the end. I've so far used this to make lasagne, and I'm going to blend what remains to give me a smoother sauce for pizza tonight.