Hi everybody !

I'm currently learning Moi and am stucked with the 8th step of this tutorial : the inward extrusion.
I've tried with the Extrude and Shell tools but it goes wrong everytime.
After a lot of tries, I have to admit that I'm rather lost and would like to have this step a bit more detailed..

Could someone please help me?
I've attached my 3dm file.


Thank's !