A few years practice will do it.
Seriously, polyrythms are hard. Depending on what instrument you're going for, you might want to try different things.
But I'd suggest just try drumming in polyrythms in your down time. Like, just tapping with your hands on your desk (at school) or just anywhere is practice.
If its for drums, also try tapping with your feet, because you'll be doing that a lot, while guitar will mostly be tapping.