At first, make sure aufs-util are installed, and please read the aufs
manual, aufs.5 in aufs-util.git tree.
$ man -l aufs.5

And then,
$ mkdir /tmp/rw /tmp/aufs
# mount -t aufs -o br=/tmp/rw:${HOME} none /tmp/aufs

Here is another example. The result is equivalent.
# mount -t aufs -o br=/tmp/rw=rw:${HOME}=ro none /tmp/aufs
  Or
# mount -t aufs -o br:/tmp/rw none /tmp/aufs
# mount -o remount,append:${HOME} /tmp/aufs

Then, you can see whole tree of your home dir through /tmp/aufs. If
you modify a file under /tmp/aufs, the one on your home directory is
not affected, instead the same named file will be newly created under
/tmp/rw. And all of your modification to a file will be applied to
the one under /tmp/rw. This is called the file based Copy on Write
(COW) method.
Aufs mount options are described in aufs.5.
If you run chroot or something and make your aufs as a root directory,
then you need to customize the shutdown script. See the aufs manual in
detail.