I started with this:
http://row40k.blogspot.com/2012/11/basing-your-miniatures-basics.html
also, youtube EonsOfBattle, Secret weapon miniatures both have very good basing tutorials (there are many others of course but those are the best intro ones
imho).
as for
I'm leaning towards Astrogranite with some snow to get the Battle of Macragge feeling.
This is how I would approach the above.
Item List:
fine sand
course sand
small rocks (or cork)
larger cork pieces (optional)
black craft paint (or Abbadon black)
grey craft paint (Skavenblight dinge, Administratum grey)
white or offwhite craft paint
PVA
flow aid (or a drop of rinse aid)
primer
wax paper
snow flock (optional)
PVA/baking soda/white paint
Get fine sand, course sand and small rocks. Depending on how much you want to pay etc, you can check your local pet store for aquarium products, craft stores like Michael's for fine sand used in flower arranging etc, or your back yard if you're lucky). You can also buy these from sites like scenic express or Hobbytown if you're lucky enough to live near one. Some
FLGS have basing supplies as well. I would not get the technical texture paint for all those bases, as it's cost prohibitive.
YMMV.
Next,
get PVA (ie glue, like Elmer's). Take your bases, if you can't remove the figures from them, then just carefully use an old brush to brush the PVA straight on. Next, set the bases on something like a piece of wax paper (to conserve the excess and keep everything tidy). Drop your largest rocks on top, then medium, then fine. Don't start with the fine, it tends to cover too much, which you then have to let dry before adding additional materials.
Let the bases dry. Several hours, I tend to set mine to dry overnight.
Now, get some watered down PVA (roughly 50/50 aprox.). Put it in a spray bottle, or you can again use an old brush, but becareful to not brush too hard and remove too much material. Give your bases a good squirting with the watered PVA. Let dry, again overnight is best (as is doing this on wax paper). This step is not 100% necessary but it helps a great deal to seal the material in and allow for ease of painting.
If you don't want to do that, skip to priming, once the bases are dry.
Either brush on or spray prime your bases. I suggest either grey or black.
Using craft paint - you do NOT need to use your good paints for this! - paint on a medium to dark grey over top, let dry. Once it's fully dry, drybrush up to a light grey. If all you have is
GW colours you'd start with Abbadon black, then something like skavenblight dinge, then adminstratum grey, or similiar. As I mentioned though. This is terrain. Just us a cheap apple barrel paints - they do just fine!
At any point during the above, you can make a black wash - or use nuln oil, but again, cost. I find you can simply use black craft paint, water and a drop of flow aid. Try the wash on a sheet to see if it's light enough (not too thick). Better to be too light - you can add more layers. If it's too dark, you will find you have to dry brush again. I tend to do this wash step after my first drybrush, and sometimes after a second, before my final.
For snow, you can use the old baking soda/glue and white paint trick, or you can buy the snow flock material. I go either way to be honest. The snow flock is pretty fun and easy to use. Just pva and glue it on. You can get fancy too and get some water effects (like Still Water from Vallejo) and make "icicles" on a piece of wax paper, then transfer it to various points on your bases.
If you added some cork in spots especially on the bigger bases, to help break them up, it'd be an ideal place to 'hang' the icicles. If you do cork in spots (for "ground" or even "broken pavement") be sure to add them first, then your rocks (rule of thumb bigger items are always added first, then smaller)
Anyway hope this helps!