I fill mine to about 3/4, but with more rendering meat on the rack,
maybe a little less is in order. The foil is a must, and do yourself
a favor and use >heavy duty< foil to cover it, not just the regular
foil. If it is full of gunk, you can simply pull off the heavy duty
foil and pitch it as opposed to tearing off the regular thickness foil
and contaminating your sand.

A couple of comments on the sand. Try to get clean, dry sand from one
of the home stores. Don't use dirty sand from the kiddo's box (also
known as the neighborhood cat box) or from a wet source like the beach
or creek bank.
I have read some hilarious comments about those practices, but only
hilarious because I didn't do them.

If you haven't tried sand in the WSM before, you are gonna love it.
My experience has been much less fuel on long cooks, and much less
temp movement than with water. On my last brisket, I went 6 hours
without even looking in, and almost 10 hours without adding fuel. The
longer I can keep that thing covered and working without my attention
the better I like it.

Robert