The easiest way would probably be to just take a used filter sponge from an established tank, and squish out all the dirty things into the new tank.
The mulm will float around, but eventually will settle in the new substrate.
That's what I do, take the pre-filter sponge and squeeze all that nasty stuff into the new tank. The pre-filter sponge has the most crap in it since it traps all the larger particles, then the smaller particles get stuck.
Looks terrible but works fine.