You know what these are: Little spiky nylon strands on a gob of silicone glue that look like grass or bits of colored foam made to look like foliage. This step is easy, just put a dot of superglue on the base and stick a tuft where you want it, easy peasy. 

If you want to make your tufts stand out, I recommend painting them. Most cheap tufts are a single color and don't look very lively. Adding a highlight with whatever color you want makes them stand out. Just add thin paint with a cheap brush in a few layers, I recommend using yellow green and then a sunny yellow color for green grasses. 

If you want to add pops of color to your tufts, tear a piece of foam into very small bits, add a bit of PVA glue to the ends of your tufts with a brush and sprinkle the foam bits onto the tufts. Paint them whatever color you'd like, and use the normal process to seal with watered down PVA. You can also purchase small amounts of colorful flock for this purpose from most arts and crafts stores.

Get your bits box out, and prime and paint up a few loose pieces. Think grenades, guns, swords, helmets, limbs, all the extra nonsense you're goint to just add to your pile of gray. If you're not using them for kitbashes, you might as well stick them around as trophies or battlefield debris. A wide variety of 3d printed bits are available online, and various retailers sell packs you can purchase. Even if you don't have a printer, most providers offer a printing service if they sell the STL files. Use superglue to put them on a few of the bases, or you can stick them in the PVA glue before you apply your basing material.  

You've built your army, and now you have a host of empty sprues to recycle. Clip straight pieces off the frame, paint them as rusty, weather worn girders, or clip them into small bits to simulate bricks. Pull gears and wires out of broken toys or electronics your kid forgot about 5 years ago to make industrial debris. ZIP ties can be turned into rails. Cardboard can be turned into corrugated metal. Old damaged window screens can be turned into chain-link fencing.  Your imagination is the only limit on what you can do with garbage. 

Touch Grass. No, really touch grass, go outside, explore. You can use a host of natural materials for improving your basing. Pull weeds out of your yard and clean off the roots. Grab loose bits of bark from the fallen tree you spot on your mental health walk. Take pebbles out of your yard and turn them into boulders. Sanitize them in the oven for 10-15 minutes at 180F to kill off microbes, then seal with PVA glue and apply to your base. If you're a child, ask a parent to help. Don't use your actual cooking trays to do this, use a disposable tray, and do not use convection ovens. 

This step is more about improving the basing you already have rather than adding to it. Think about where the light would be hitting your model, and use washes, glazes or watered down speed paints to add shadows beneath your model. Use a bright flashlight to help you figure out where your model would be blocking light. 

If your base is not popping as much as you want it to, drybrush a lighter color that suits the material over the base, pick out larger grains or pebbles and paint them an entirely different color. The real world is alive with color, light and shadow, so your bases should reflect that. 

Pigment powders are a powerful tool for making your models look like they are where you've based them. There are a lot of options available from hobby companies, in a lot of different colors. If you don't want to order them or can't, you can use art pastels (acrylic, not oil) and scrape them with your hobby knife to create powder and, using the crappiest brush you own,  just brush it onto the base, and onto your model's lower half, focusing on the feet. Once you seal your whole model with varnish, it won't budge. If you want to make it look like thick mud, apply watered down medium or varnish to your model before applying the pigment, as it will cake on the surface and seal it down.