Well, if you're strong with rhythm guitar playing then you'll have an advantage already. Start tapping to everything you hear - try to make yourself a virtual drumset (this is what I do), like two feet play the single or double kick, right knee area is snare, left knee area is hi hat, thighs are toms and cymbals are invisible.
Really though try it and try to really nail the exact groove of each song like you would if you were playing rhythm guitar. Drums of course are more mechanical if you will in metal and music in general then rhythm guitar so think of the precision needed to play, say, some Dave Mustaine or Chuck Schuldiner type riffs and palm muting and such. The really really rhythmic, defined stuff. When you start feeling comfortable with it stretch yourself out a bit and see if you can improvise or improve or alter the drum parts of whatever selected song and have it still work. This helps you gain your own feel for drumming and drum nature as well.
And of course listen listen listen listen. A lot of people don't really take the time to open their ears to specifically what the drummer is doing in a band, this is one reason drummers are underrated. Personally I prefer players like Thomen Stauch (Blind Guardian), Horgh (Immortal), and David Husvik (Extol), guys who have incredible precision as well as an amazing amount of feel. And they're all produced well so we can hear what they're doing.
Hope this helps. BTW I'm a guitarist too.
