I've done this before and it is very simple, but you do need a blender. It works in a pinch but I'd much rather just buy a carton of it.

The problem I've found is that it's very tricky to filter properly. If you don't filter it at all then you end up with a grainy product, but it's far too thick to go through something like a coffee filter without clogging it up so you need to use cheesecloth.

Another problem is storage. Making it in small quantities as you need it is fine as long as you're ok with it being room temperature, but if you want to make enough to keep in the fridge then the oats are going to begin to separate from the water almost immediately unless you add an emulsifier.