Hi
There's loads of advice from the NHS about making up feeds. The current advice is to make up feeds when you need them (as the powder should be mixed with water that has been boiled and that is still over 70 degrees - so that any potential germs in the powder, which isn't sterile, are killed). As suggested, the best way to do this when you are out is to take boiling water in a flask, which you can add to the powder when you're out.
Having said that, not everyone finds this practical....plenty of people do it the way Heatheror12 has suggested, and not had any problems.
We used to make them up as per NHS advice (with boiling water) but then cool them quickly in iced water and put them in the fridge. We then warmed them in hot water when our daughter needed one. However, if you do it this way you need to make sure they're made up fairly freshly i.e. we always ensured we used them within half a day......don't leave them in the fridge for days!!!
Hope that helps
xxx