Water for Formula

My LO is 6 weeks old. I am very cautious with pretty much everything. I still boil water to mix with my formula and prepare 8-14 ounces ahead of time. (he gets breastmilk mixed with it).

The pediatrician said I could use distilled water but just hesitant due to the formula powder not being sterile in the first place. However, he does not have any immunity concerns that we are aware of to be this cautious, I guess. 🤔

How do you mix your formula? And how do you warm it up?