Recently I was asked to throw up some basing tutorials, so here we go, such things I have learned about basing as you may find useful. I'll be doing a few of these, I imagine.
1. The Porridge
I've already posted on this, you can read about it here. However, there's one element of the mix I didn't discuss, and that's crushed shell.
It's fantastic stuff for mimicking shale, mixing in to a
basing mix as I'm do and for using larger pieces on bigger bases.
You can also but it on-line I imagine, but it's easy to make. Get thee to a
beach, get a shell or two, and when you get them home, wielding a
hammer, pulverise it. Make sure to wear goggles, and do it somewhere
where flying shards of razor sharp shell aren't likely to take out the
cat/child/postman. Take the resulting debris and marvel at the
marvellous bits of basing material you've just made.