3/4 Cup of Warm Water (plus some to add to achieve correct consistency)
Using a stand mixer with a whisk attachment, pour all of the sugar and meringue powder into a bowl and mix on low until incorporated.
With the mixer still running, pour in 3/4 cup warm water.
Stop the mixer, scrape the sides of the bowl and then turn the mixer on medium for 6 minutes.
After that, turn the mixer on high for 4-5 minutes. The icing should be very stiff at this point.
When working with the icing, make sure to keep the bowl covered with a damp towel to ensure that it doesn't dry out. If you put this icing in an air-tight container, it should save for weeks (just make sure to give it a good mixing before using it).