Here's what I do to make my roasted tomato sauce (which is closer to a paste, really).
Select a good assortment of tomatoes and wash.
Chop roughly and put the tomatoes into a glass roasting pan - I generally use an 8 x 13" pan. I drizzle olive oil over them and then add some Italian seasonings. Some people add onions, garlic, or other herbs. Be creative - do as you like!
I set the oven to 400 F and put the pan on the middle rack for about 2 hours, turning the tomatoes with a spoon now and then, until the liquid is considerably reduced. Here's how they looked when they came out today. Keep an eye on them - you don't want the top layer browning too heavily because it will give a burnt, acrid taste to the paste.
After they are out of the oven, I let them cool for a while.
I have a stainless steel manual food mill. This was very handy when I was making a lot of tomato paste, and is great for other vegetable purees and fruit purees.
I had an electric food mill for a while but it was cantankerous and inefficient - I much prefer this manual one. Mine has 3 different sizes of metal plates for the bottom that allow different sizes of particles to pass through. I use the one with the smallest holes for tomatoes because I don't really want lots of seeds.
If you don't have a food mill, you could crush the tomatoes with a potato masher or forks, and then use a medium-mesh strainer to achieve the same end product.
I place the food mill over a bowl and pour in the tomato mixture.
After some elbow grease has been applied, the remainder in the food mill is just skins and seeds (which the chickens will get as a snack in the morning!)
The resulting tomato sauce can be further reduced if you wish to make it even thicker, or it can be frozen immediately. This stuff is wonderful - it is the pure essence of ripe tomato, concentrated and thickened into a dose of summer.
I freeze it in freezer bags because it makes for very efficient storage. Squeeze the air out of the bag when it is laid flat, so that the paste is almost at the top. Lay flat in the freezer to freeze, and then once frozen, the bag can be placed on its side or end in a "library" of bags. I used to make paste with red, orange, yellow, and mixes of tomatoes, so I always had many colours of paste to choose from.
It's great in winter for lasagna or other pasta dishes, soups, stews, and almost any recipe where tomato sauce or paste is used. Give it a try if you have a lot of tomatoes on hand!