Formula should always be made with recently boiled (hot but not scalding) water. I usually boil the kettle, pour it into the bottle and let it cool for about 5 minutes before adding the powder. The reason being that you boil the water not to kill any bacteria in the water itself, but in the formula as it isn't sterile (and in rare cases, babies have died from formula that wasn't made with hot water). You can however made several bottles at once, cool them quickly (sitting in a bowl or sink of ice cold water for a few minutes until cool to the touch), and store them in the fridge until you need one. It's recommended to make them fresh, of course, and that's what it will say on the packet, but making it that way is still safer than making a bottle with cool water.
It's not recommended to heat them in the microwave (it can make hot spots, and microwaves do affect some nutrients in food, so probably do in formula as well). But people do it. I find just sitting the bottle in a bowl of hot water brings it up to temperature in about a minute or so and that's what I prefer.
And just as a word of caution, which I'm sure you already know about, but supplementing can affect your supply. I'm guessing your LO is a bit older (not a newborn), so your supply is probably better established. But I had to supplement with formula for health reasons at the time (I was sick and my daughter was having trouble latching, so I had to switch to expressing with some supplementation because she was losing weight). Even with expressing hours and hours a day, I couldn't keep my supply up after I started to supplement and it dried up completely. Now that's probably partly due to a low supply anyway and that I was unwell, but just be careful if you do notice your supply drop and you would like to keep BF at least some of the time.
