Hiya
It says on the packet to use hot water but I don't use hot I use warm - usually boil the kettle and leave it for a little while to cool. Put the water into a bowl once you have measured the water out of course, put the powder on top and I use a fork to sort of whisk it.
If I use hot water, it seems to get a lot more lumpy and clumpy and the lumps of powder aren't that nice for me. I prefer it much smoother and when most of the lumps are just the oats.