To easily fill zip-top bags, stuff the bag into a wide-mouthed pint or quart jar. Fold the top of the bag down over the sides of the jar and spoon your soup, sauce, peas, or whatever you're working on into the bag. Fold the top back up, remove from the jar, and seal.
To 'burp' the bag, leave a tiny bit unzipped; slowly slide the filled back over the edge of your countertop. Excess air will be expelled, leaving you with an air-free bag.