The trick is to get a thin uniform layer of the heatsink compound applied to the bottom of the DIS. More is not better in this case.
Clean the surface that the DIS mounts to ( I actually polished mine)
make sure its good and clean.
Check the bottom of your Dis and make sure it is clean and check for flatness.
I have had to level the bottom of some modules (draw file and polish)
Get it good and clean.
Apply the heatsink compound to the DIS and smear it thin and even ( I use an old credit card to spread it even.)
Clean the threaded holes that mount the DIS, and clean the threads on the fasteners that mount it.
Put the DIS in position and put the fasteners in finger tight, then fasten it down a little at a time to get the torque even.
This might seem like over kill, but it's better to error on the side of over kill when dealing with a DIS module.
(FYI I use artic silver heatsink compound--which could also be over kill, but It gets the job done for me )
Others may have different advice, so pick your favorite!