Create the directory for storing your rootfs data. This could be a loop mounted dir, or just a plain directory. For e.g:

mkdir /destination/

Install the baselayout (contains the basics of the filesystem)

pisi it --ignore-comar baselayout.pisi -D /destination

Install all the pisis you need (replace abcd.pisi with yours)

pisi it --ignore-comar abcd.pisi -D /destination

Next we need to configure the rootfs. For this, we need to first setup some requirements:

mount --bind /proc /destination/proc

Configure the package

pisi cp baselayout.pyp

pisi cp

Generate a squashfs

mksquashfs destination myimage.img -noappend -comp gzip -ef /tmp/lkj