Chop tomatoes up in quarters, put them into a blender accompanied with the basil, oregano and some salt. In case of pureed tomatoes just mix the ingredients together.
Blend them mildly.
Pour some oil into a saucepan and heat it up. Add tomatoes in then bring it to boil.
When the first bubbles appear turn the heat to low. Place a lid on halfway and simmer for half an hour. Stir it occasionally.
Once the water content is reduced by about half the sauce is ready.
Advanced
Pour some oil into a saucepan and saute onions with salt on high heat until the onion gets a translucent / glassy look for about 3 - 5 minutes.
Reduce the heat and slowly cook the onion while stirring every now and then until it starts to caramelize for about 15-20 minutes.
Pour pureed tomatoes into the saucepan and bring it to boil. Also, now is the time to add various stocks or wine to the tomato sauce if desired.
When the first bubbles appear turn the heat to low. Place a lid halfway on top and simmer for half an hour. Stir it occasionally.
Once the water content is reduced by half stir in the finely chopped spices like oregano and basil. Bring the tomato sauce to boil one last time and let the flavors come together while it cools.