To get tallow for soap, have a food stockpile near your kitchen that does not take the fat category (which includes fat, tallow, wax, and pudge). It should take all the other food you want to go into meals. Basically everything but fat, drinks, seeds, lye, unprepared fish, prepared food, and maybe plants. Then create another stockpile farther away (over by your soapmaker's shop) that takes the fat category. Now when something gets butchered, the fat will get taken to the fat pile, then a cook will bring it back to the kitchen to render to tallow, which gets taken to the fat pile. Now when your cook wants to make a meal, he'll grab whatever is closest, which will not be the tallow if you have anything else that can be cooked.
Optionally, for a bit more efficiency, you can allow just the fat in the kitchen stockpile and just the tallow by the soapmaker's. But that involves going through the fat category in the kitchen stockpile selecting all of the fats and not the tallows. Then do it again by the soapmaker's, selecting all of the tallows. And there are hundreds and hundreds of entries, generally two for each type of animal, and there are a lot of types of animal (how many different types of gibbon does this game really need? it's got at least 5 of them). But you can't really do it with a macro, because a few animals are different. Bees (honey and bumble) have fat, tallow and wax. Fluffy wamblers just have pudge. Mussels and oysters have fat but no tallow. And every forgotten beast in your world is listed, so you'll have pages and pages of:
forgotten beast fat
forgotten beast tallow
forgotten beast fat
forgotten beast tallow
forgotten beast fat
forgotten beast tallow
So basically, it's best to just accept that the fat will have to travel to the soapmaker's and back before becoming tallow.