suck it

Fill the colonized substrate in a opaque casing container, level it with a clean spoon or fork and spoon the casing material on the top of it as even as possible, without pressing it down.
Don't try to create a completely flat surface, small valeys in the casing surface are beneficial and necessary for primordia formation.
The thickness of the casing layer should relate to the depth of the substrate layer ( deeper substrate layer -> thicker casing)
It should be around 1/4^th of the substrate depth.
So if your substrate is 4 inches deep, the casing should be around 1 inch thick with a limit at somewhere 2 inches.
So even if your substrate layer is thicker than 8 inches, a casing layer thicker than 2 inches is usually not applied.

A quote from Paul Stamets and J.S.Chilton: The Mushroom Cultivator:
"The correct depth to apply the casing layer is directly related to the depth of the substrate. Greater amounts of substrate increase yield potential which in turn puts more stress on the casing layer. Prolific first and second flushes can remove a thin casing or damage its surface structure, thereby limiting future mushroom production. A thin casing layer also lacks the body and moisture holding capacity to support large flushes.
AS A GENERAL RULE, THE MORE MUSHROOMS EXPECTED PER SQUARE FOOT OF SURFACE AREA THE DEEPER THE CASING LAYER.
Agaricus growers use a minimum of one inch and a maximum of two inches of casing on their beds. Substrate depths of six to eight inches are cased 1 1/4 to 1 1/2 inches deep. Substrates deeper than 8 inches are cased 1 1/2 to 2 inches deep. Nevertheless, experiments in Holland using casing depths of 1 inch and 2 inches demonstrated that the deep casing layer supported higher levels of microorganisms and produced more mushrooms. (See Visscher, 1975). To gain the full benefits of a casing layer, an absolute minimum depth on bulk substrates is one inch. For fruiting on sterilized grain, the casing need not be as deep as for fruiting on bulk substrates. Shallow layers of grain are commonly cased 3/4 to 1 inch deep."