for songwriting you need a bass will pulse at a beat with the drums and flow with the vocals. The drummer needs to kick back in the intro and then pound the chorus to shreds. For guitar you dont neccesarily need a lead player, but two is always better than one. If you lean more on the style of nirvana, then on your amp, you need to turn your treble and bass at max, and the middle at zero. Then turn you distortion at about 80% during the chorus and about 15% percent during the verse. Use mainly power chords, palm muting, bending strings during solos, and last but not least puts some melody in it,Dont feel like you have to be super heavy, because then its not even music anymore.Try just jamming, thats the best way to write a grunge song.
bands like audioslave, foo fighters, stone temple pilots, and silver chair are good, and 90's, but THAT DOES NOT MAKE THEM GRUNGE. listen to , motherlovebone, temple of the dog, green river, mudhoney, the melvins, screaming trees, and other pre 90's grunge. If you get the roots for the more popular grunge bands you can learn how they got their stlye. And also listen to some non grunge bands like pixies, vasolines, black flag who inspired nirvana and others. If you want to real, then listen to the underground. And let me clarify something stone temple pilots are just pearl jam clones, theyre from san diego not seattle.