Sofrito is a puree of ingredients used to flavor many latin dishes. I use it often to make rice and I have also used it as a rub on chicken before roasting. The taste is wonderful. It just imparts flavor to your dishes. It is simply an aromatic puree of bell peppers, tomatoes, cilantro, garlic and onions. There really is not a certain amount of each vegetable to use. If you like it more garlicky, add more garlic. If you love cilantro, by all means add more cilantro. Make it to your liking.
I use a blender to puree my vegetables. I find that I have to add a couple of tablespoons of water to get the vegetables to blend in well.
I store the sofrito in glass jars. I then store these jars in my freezer. When I get ready to cook a recipe that requires sofrito, I let it defrost a bit and scoop it into my recipes.