Daltons Aquatic mix is rather like fine soil, and as soon as you add water, it turns to mud, so washing it is going to waste a great percentage, and nigh on impossible.
I just chuck mine in, then put the stones on top. Put a plate or saucer on top, then pour the water onto this to partialy fill the tank to about 60mm. Remove the plate and position and insert your plants into the substrate.
When this is done, put the plate/saucer back in and carefully fill the tank with water. The purpose of the plate is to stop the water gouging holes in you substrate and allowing the Aquatic mix to end up floating around in the tank, and ultimately, when it settles, on top of your stones.
Needless to say, no mater how careful you are, a bit of Aquatic mix clouding the water is inevitable, and it will settle after a while, and you can vacuum it out.
HTH