There are several ways to prepare your tomatoes for making sauces. I will mention a couple of way here.
1: Blanching
Clean your sink and fill with water. Add about a cup of white vinegar to wash your tomatoes in.
Fill large stainless steel pot about half full with water and bring to a boil. Cut an X in the bottom and top of each tomato.
Working in small batches, immerse tomatoes in boiling water for 30 to 60 seconds or until you see the skins start to split.
Immediately transfer them to ice water.
The skins should slip off easily in your hands. These are now ready to can whole, cubed, quartered or crushed.
2: Using a Food Mill:
This method will remove skins and seeds. Wash and quarter the tomatoes. Put them in a stainless steel pot and begin cooking over low heat. Continue adding tomatoes and mash them with a potato masher. Cook until softened. I usually do mine in a roaster and cook them overnight. Use a stick blender to blend the tomatoes. Press them through a food mill to remove skins and seeds. There are several different types of food mills. I have a Foley and a Squeezo...I love them both. Can sauce as is or use in favorite recipe.