Once we really got into the swing of things we used to make up a bottle with the powder, boiled water added to the powder and swirled to mix, and then cooled it with pre-cooled boiled water. I probably didn't do this until E was older and I worked out the amount of water needed with the powder already in the bottle but it really helped.
I also had a flask for carrying around boiled water for a couple of hours and used pre measured bits of powder.
I would imagine the pre mixed would be much easier for the early weeks (you could always use a flask to warm it if also isn't keen on it cold.
I remember my feeling of deep dread when I realised DD wasn't managing with BF. I remember my friend coming over and talking me through the process 3 or 4 times before I got it, but once I got used to be routine of sterilising etc it was easy really.