I mixed mine up before I put it in as well as I could without dumping it all into another bag and doing the shake and bake thing...
I am not sure if this is standard practice, but common sense said I would rather have all the material somewhat evenly distributed so to that I could get good moisture retainage and drainage throughout the substrate.
How's that for an oxymoron.
Someone with more experience could probably give you a more concrete answer, but that's what I did.
When you put it in, I like to fully soak it first, then squeeze it out, handful by handful as you put it in. Otherwise, you will find that it seems to be very reluctant to soak up the water the first time.