do you have a photo or a link to what they look like? If they're the typical cheap blowmold plastic, then the best bet is to use a combination of paint - either the more $ plastic spray paint, then follow up with hand-painting in details using either craft enamels and a small brush/sponge or go with really thick acrylic paint (but that will likely be prone to flaking off eventually). You'd need a few different colors if trying to simulate wood, but may be easier to just paint a flat black and then use some metallic paint to highlight any metal connector parts (like bolts/screws if they have those). If wood grained, then paint a flat black, then use a sponge to gently stipple a medium brown onto the high points, let it get tacky dry and then use a clean cloth to lightly buff it to remove some of the paint/smooth down. Then use a lighter brown and hit some of the high points where you'd see the handle be worn down if used regularly. Smooth lightly to blend. Can repeat several times with different colors of brown to achieve a realistic, worn/used wood tone look.
For the blade part, you'd need something like rubber cement/glue on the blade, then take a length of aluminum foil and carefully fold it onto the blade area, making sure to not get it wrinkled or creased for the best effect. Wait until completely dry to trim off excess foil and smooth it down well.
But overall, keep in mind how and where this is going to be viewed and adjust the amount of effort/$ accordingly. Just painting it a dark brown/black for the handle and putting the foil on the blade is going to be great for lower light situations and most everyone that sees it will think "that's supposed to be a real knife" and since everyone knows you'd never use a REAL knife anyway... good enough likely could be good enough.
Frankie's Girlis right on the mark with her suggestions. Roughing up the knife with a bit of sandpaper will help the paint stick a bit better. I found that just trusting the "good for plastic" spray paints didn't hold on that tight without a bit of sanding to secure things. I used sandpaper that was on hand with a grit that was a bit too aggressive. But after four years of being out in the weather, the paint is still there. A scythe I painted for my grim reaper that I didn't sandpaper, peeled away after the first year.
I just used a silver paint and highlighted it a bit with gold for the blade. The handle is pretty much following Frankie's Girl's advice right down the line. You can see the handle, as the hand wasn't adjusted at the time the picture was taken. I have to admit that the foil idea never occurred to me and I might have to try it in the future. But for a Halloween night out in the dark, the blade stuck in my pumpkin's head looked just fine.