If you are using a pourable grout, form two opposite sides of the base plate
and build up retaining curbs beyond the other two sides. Fill the space by
pouring the grout into the space between the plate and the retaining curb on
one side until it emerges from the other side and rises above the bottom of
the plate on both sides. In that way, most of the air will be swept out
from under the plate as the grout moves from one side to the other.