Ok, I am going to try my best to make this "not" sound like stereo instructions. I will go start to finish, with some deviations as we go. Of course my bases will not have models on them, but you will be able to get the idea once you see them.
Step 1: Painting the BasesI always start this by painting the bases. In the picture I have painted 6 bases. Three of them are VMC Beige Brown, and three are VGC Charred Brown. Here is an important tip. It does not really matter what color you paint them. Choose a color that looks like some kind of dirt or whatever your ground is. You won't be able to see a whole lot of it, and it is really only there to cover in case the sand misses a spot. You could probably even get away without painting them, it is just what I have always done. When I first started painting miniatures, the fella I learned a few tricks from actually used the paint to adhere his flock to the bases. I would not recommend that, but I know it worked for him.
Step 2: Glue and SandOk the next step is to glue the sand down to the base. Lets start with the sand. I know many people that just use playground sand or ballast for this. There is nothing wrong with that. On the other hand, I have a mix that I use that includes several different sizes of sand, ballast, and small gravel so it really varies the way it looks. For glue I just use PVA glue (Elmers to most). I usually water my glue down about 3/1 water/glue. That makes the glue easier to work with. Get and old brush. One you don't really care about. Use it to spread the glue around. Be careful not to get any glue on the model or it will pick up sand, but don't worry much if you do. You can always come back after it dries with a damp cotton swab or toothpick and remove it.
Now, cover the exposed area of the base with the glue. Then dip it in the sand. Let is sit in the sand for a second so the sand can get a chance to stick. After that, remove the base from the sand. Turn the base upside down and tap the side of it with your brush to remove any loose sand. Set it aside. It helps if you do several models at the same time.
Step 3: More Glue!!
I know we already did the glue step, but trust me this will be needed. Once your original glue dries, we need to add more. See, all that lovely sand that is stuck to the base is just waiting to come flying off the minute you put the model in your case. So to make sure it does not, we need to basically encase the sand in glue. Don't worry. Thin your glue down a lot! I mean A LOT! You want the glue to be the consistency of a thawed out milk shake. Once you get it that thin, go ahead and dab it on the base. You will need to use that old brush again for this step. Dab the brush onto the sand, but try not to actually run the brush across the sand. This will keep you from moving loose pieces that did not glue well. Don't worry if it looks too thick, it will dry clear. Make sure that all sides of the little pieces of sand are covered in glue, as well as any larger pieces that have stuck there.
There is another reason for the glue in this step other than just holding the sand down. See, once the second coat of glue dries I am going to have you wash the base with some of GWs wonderful wash. The sand you glue to the base is just like a model itself. Even though it has curves and natural shadows, those shadows are often not very noticeable with out a little help. So for the same reason we paint a model with highlights we will need to get the same effect on the sand. If the sand is not coated in glue the wash will be absorbed by the sand/gravel and all be the same color. With the glue on there the wash will settle and will give you nice definition.
Step 4: Wash
Now that you have the glue down and dry. We can move on to washes. The three bases in the picture above have each been given a generous coat of wash. From left to right the colors used are Gryphonne Sepia, Devlan Mud, and Ogryn Flesh. Notice how much more depth they have than before the wash. I thought this would help you see what your options are. This step is pretty straight forward. Put the wash on your bases and let dry.
Step 5: Finishing Options
As you can see there are several options you have after you use the wash. You could leave the bases as is and go with no foliage. You could also lightly dry brush the bases with a light color. In the pictures above I have chosen Static Grass (Top) and Flock (bottom). I typically prefer the look of the static grass but I have included an alternate idea so you can see a comparison.
Applying the static grass is a fairly easy task. Apply glue to the areas of the base you want the grass to cover and then dip into the container with your static grass. You can cover as little or as much of your base as you would like, it is completely up to you. There are a few tips to making the static grass look a bit better. First off, try not to press the grass into the glue too much. This can cause the grass to lay down and look matted. Just dip it into your container, let is set for a second to give the grass a chance to stick, and then remove it. Turn the base upside down just as you did with the sand and tap the side of it to help remove any extra. Another step I like to take with static grass is to blow from under the base on the grass. This will help it to stand up and keep it from laying down.
Applying the flock is pretty much the same steps as the sand above. The main difference is that you will not need to apply a second layer of glue.
Step 6: Enjoy, and Mix it Up
That pretty much does it. You should have some nice looking bases to complete your hard work on your miniature. Give these steps a try and don't be afraid to mix it up. The pictures above are the same steps we followed above, after the flock I added some static grass to the top. That gave them even more depth, and added a touch of realism to them. You can even have a try at some other materials as well. I have seen basis where the painter used dried spices to look like leaves that had fallen from a tree. There are many things you can do, have fun with it. I hope this info helps you. Enjoy.