There are two things for you to think about - the government advice, and what is practical for you.
Government advice is to make the bottle up as you need it with water at least 70 degrees (to kill any bacteria in the powder), which means from a kettle boiled no more than 30 mins ago. You should make up one bottle as you need it, and not store them. Anything that you've made up should be thrown away after 2 hours. You can use a flask to store the hot water, but personally I found this a nightmare. There are various problems with adhering to the guidelines, not least that the tub instructions want you to leave it the full 30 mins before making it up. I suspect this is because the steam from very hot water clogs up the scoop and you get too little powder! By following these guidelines you could have a screaming baby waiting for 45 mins while you get it sorted - not good!
Previous guidelines have suggested storing cooled boiled water and adding the powder when needed, or making up the bottles, cooling quickly and storing in the fridge for no more than 24 hours to warm as needed. I've also known people to make them with boiling water, and keep them hot in a thermal bag. In any case, once a bottle has been in your babys mouth, you should limit how long you keep it for. Some people say 1 hour is ok, others 2.
You should follow whatever method you are happiest with. None is 100% problem/risk free but people have been using all of them for years with only a tiny number of problems. Personally, I store bottles made up in the fridge, I make two batches per day - one for the day and one for the night, and generally don't store them for more than 12 hours.