Step one: install the SDK on your system
Step two: add deb-src deb-src http://http.XX.debian.org/debian/ etch main contrib
into /etc/apt/sources.list.d/debian.list replacing XX with your closest mirror.
Step three: apt-get update
Step three and a half: cd $HOME
Step four: apt-get source shrew
Step five: cd shrew-* && dpkg-buildpackage -rfakeroot
Step six: if everything went wel you should have gotten a shrev_$version*_armel.deb in $HOME which you can now install on the N900
Step seven: copy the deb over to your device
Step eight: dpkg -i $DEB

done...