I plan on basing my figures using the Army Painter brown battleground rocks (the tiny ones) + static steppe grass flock and some highland tufts scattered about.

The question I have is -- when doing the rocks + grass flock, should I cover the whole base in a layer of rock first and put the grass over the top as second layer (after the first dries) or should I do the grass directly on the base?

Any advice?

I havnt seen or used the rocks you mention so its hard for me to advise well...but when I base, I think it looks better if you do the whole base (certainly with mud etc) then grass over the top of that.