Disclaimer: The following stuff is illegal. If you get caught by the smog police and sold into slavery in Mexico for your crimes, you didn't hear it from me. Seriously, this will significantly increase your car's output of raw, partially combusted, toxic as hell hydrocarbons. You're a bad person for even reading this.
OK, now that I've got that out of the way, here's how to install breather filters:
I am not going to go into a discussion of what the engine's crankcase breather system does, but it is essentially a way of getting rid of the fumes and vapors released by the oil in the pan as well as blowby from the cylinders and the expansion of the air in the crankcase as it gets hot and expands. On cars that have a MAF sensor to meter air, the crankcase system is basically sealed and doesn't allow unmetered air to get into the intake manifold. It is important to maintain this second part. On most Nissans, there are two vents that come off of the valve cover. One is the "fresh air inlet", that goes from the intake piping to the valve cover, and the other is the PCV valve, that goes from the plenum of the intake manifold to the valve cover.
--Take the two hoses off the engine.
--Remove the PCV valve and cut the flange off the end that holds in the check valve--you don't need it anymore and it will just impede the flow of fumes (which is bad). How you do this depends on the design of your particular PCV valve, but it should be obvious. If you screw up, a new valve cost about $2, so don't sweat it. You just want a hollow fitting left over.
--Reinstall the PCV valve.
--Put a breather filter on each of the two vent tubes on the valve cover (one of which should be the PCV valve).
--You now have two holes in your intake tract - one on the intake pipe and one on the manifold. Get some rubber vacuum caps from AutoZone and securely plug these two holes. Make sure they are airtight and not going to fall off. Otherwise, you have a vacuum leak and your car won't idle and will run really lean under throttle.
--Sit back, crack open a cold one, and savor the feeling of spitting in the eye of 35 years of emissions regulations.