replacing door lift cylinders/miniature shocks/gas springs

I have a 1997 850 GLT liftback and the rear door cylinders no longer work so that the rear door won't stay open on its own. I am about to purchase the replacement parts (they are called either door lift cylinders or miniature shocks or gas springs) but I don't know how to install them? The existing cylinders seem difficult to get to ...does anyone know how to replace them?

Thanks,
MG