I use 2 5G buckets from Home Depot. I mix with RO
thats sitting in a 'Brute' 32G (In my kitchen, wife hates it ; ) I mix until the SG
reads 1024 or 1025. I don't get too picky about it since I don't use a refract and most hydrometers are a little off. I use about 2.75 cups of salt to 5G of water, let it mix (via powerhead), do the next bucket, let it mix and then drain the sump (Shut off the return) of 10G of water. Then, simply pump in the good stuff and restart the return pump. Others adjust temp and mix for hours to days ahead of time but I haven't seen any negative impact. I would not do this with larger water changes since it would obviously cause a tremendous change in temp.
As a side note I just switched to Oceanic Natural Sea Salt Mix and the corals look 2x as good as they do when I use IO. (Little experiement I doing..)