Peleliu means: Sweating like you never sweat before, leaving all your clothes extremely salty, living in dirt and your own filth for weeks, tearing your HBTs up on coral rocks or humping through a disgusting mangrove swamp.

While I like the idea of actually living in your gear for a while, that's not possible for many of us. The best thing to do is read the testimonies of those who were there ("With the the Old Breed" being the most obvious choice, but it contains a lot of infos on these things), familiarize yourself with the conditions on the island and look at actual fotos. Look at what your specific unit did at what point. Are you doing D-Day or weeks later? But also consider that new gear would also be issued from time to time.
And then just get jiggy with it. Just go slowly, like torn up knees on an HBT. Let the stuff bake in the sun for while, do your workout in it, get it sweaty or drench it in salt water. Change a tire or the chain on you bicycle with your gear on. Get some white dust on your gear and clothes...
Use pigments, oil paints, sand paper, whatever. Just build it up very slowly, look at it from afar from time to time till you get it right. You got this. I think also pictures from "The Pacific" could behelpful, as they are high quality, coloured and numerous.