I guess you don't really, you are just guided by them and what they want. I began weaning mini-Nat at 5months or thereabouts, and it began with literally just tastes of food, no more than a teaspoon full. I would always put about 2 desert spoons in a bowl, but she would rarely take more than a couple of baby spoons / teaspoons. Gradually her appetite increased until now (at 10months) she no longer wants bottles (although I give her one before bed) and has four small meals a day. She has a big appetite and is a big baby, but you will know yourself with your own.
Also, if you intend to make your own food including first foods, then a good idea is to make a batch (pureed veg, fruit, etc) and freeze them in ice cube trays defrosting them as and when you need them. I would say that one ice cube size is a good start for when you first start weaning.