I did it exactly this way too, although I waited for kettle to cool for more like 20-25 minutes, due to making up more than one bottle and not wanting water to have cooled down too much for last bottle. I might have been worrying unnecessarily on that point though! I ensured all the powder was in each bottle while kettle cooled then had a fifth sterilised bottle in which to pour the right amount of water from kettle and then transfer this to a bottle with the powder before adding lid and giving a quick shake. After making up all 4 bottles in this way I ensured I shook them all again more thoroughly before transferring to container of cold water, flash-cooling for about 10 minutes, then transferring to top shelf of fridge, which I cleaned regularly and kept clear of anything else.
OP, you have to chuck bottles away 2 hours after making them up and attempting to feed baby straightaway i.e if they’ve not been stored in the fridge. But if you follow @PinkMoscatoLover’s process to make up bottles, then flash-cool in cold water and transfer to fridge, they can be kept for up to 24 hours. To be on the safe side, I did make up 4 bottles in the morning and 4 bottles in the evening and ensured anything past 12 hours was chucked away. I did find I never actually needed every bottle but it was reassuring to have enough made up in the fridge.
At the very beginning, when baby was drinking little amounts and often, I did use ready made cartons (having struggled with breastfeeding for first two weeks), just while I found my feet and researched a safe way to make up bottles in advance.
I was happy that my method was safe, but I was quite OCD about sterilisation, water being right temperature, correct timings followed to a second, so it did feel a bit like a military operation! I was fortunate in that DD was quite consistent with her naps so I could make them up in the morning and dad was able to look after her if she was unsettled while I got night bottles done! When DD was coming up to a year old and had dropped to 3 bottles a day, I switched back to ready made cartons, having worked out that Child Benefit covered the cost, and also my method was getting too much while also weaning!
A prep machine would have been easier, no doubt about it, but, when we looked into it, we came across NHS studies that suggested the hot shot wasn’t enough to kill all of the bacteria that may be present in the formula powder. I couldn’t unread that once I’d seen it and I just couldn’t take the risk. I read about babies in other countries very sadly dying because of contaminated powder that hadn’t been sterilised correctly when making up the formula. Having had a miscarriage already and going through a very stressful pregnancy, I absolutely was not taking the risk. I read a lot about how to safely make up bottles in advance before I started doing it and was much happier with that method, and DD was fine 😊