It should be pretty straightforward, just note that depending on which version you want to build the arms have an optionnal piece.
If you want to build a closed up devastator/demolisher you should skip the piston bit, if you aim for an "open" one or the spriggan you should use it.
I've found this guide (
bols alas) which might clear up some things even if you don't plan on magnetizing.
http://www.belloflostsouls.net/2012/11/hobby-magnetizing-demolisher-devastator-spriggan.html