Rootfs etc/init.d scripts

hai frnds i need help .i want to create root filesystem using busybox. am using
riscv64-linux-unkown-gnu- make menu config
riscv64-linux-unkown-gnu- make install
after did these steps i got rootfs it contains only few directories 1) bin 2) sbin 3)linuxrc 4) usr
but if we want to run this rootfs based on our board we need to add extra directories that means mount,home,dev,proc,sys,etc.
inside etc/init.d we need to write based on our requirement some scripts we need to write .
if we boot our board first it will go proc and init.d scripts ,but i am confusion how to write these scripts i am using ubuntu 16.04 ,so please help me if any body knows.