From the FAQ sticky thread, go to
www.isuzufaq.com and look at item #5. This gives instructions and links to more instructions and pictures for the manifold gasket replacement.
I started this on my '99 for a leaking gasket and ran into some issues. You might want to monitor my thread
Detailed help needed with IMG replacement on '99 Rodeo 3.2L for any advice given there.
FWIW, I bought the complete IMG set (plenum and two lower gaskets)from rockauto.com for $25 + $8 shipping.
Hope it goes smoothly.