Are you doing a water change or filling a new tank for the first time? It sounds like you are filling for the first time.
If you have the means to mix 24g at once, great, but if not, and you do buckets, you just need to measure the SG of each bucket you make up and make sure it's the same. On an initial fill, there isn't any livestock to harm, so as long as the water is as close as you can get it from one batch to the next, you'll be fine.