You buy some cheap toy plastic figures from a charity shop or car boot sale – or swipe them from any kids you know – and spray undercoat them with whatever primer you like.
I keep these handy for experimenting on and they’ve proved very useful for quickly trying out things – shades, washes, paint density – particularly with the airbrush.
You can do this on a piece of paper, of course, but nothing beats a 3d object.
It’s closest to doing it ‘for real’ – a no risk way of trying out techniques and ideas. And when you’ve used up a dummy, you can chuck it, strip it or re-prime it again . . .