I recently used a Vallejo dark green wash from the bottle and loved the effect when applied over the top of a pre-shaded model. I now want to do a similar method on a sand coloured model, but will need to make my own wash. Is it as simple as mixing some thinners to thin down the colour? I've also heard of thinning with water as well as Future/Klear... any advice?

You could give glaze medium a shot if you have any. I've been meaning to try this out as well at some point. I think a combination of thinner, glaze and then thinning with water might do the trick. I have the sneaky suspicion that glaze medium is just heavily thinned acrylic medium, at least for the most part. A little googling might shed some light on the matter.

Kimmo

I use glaze medium quite a lot as it comes in very useful when painting figures. It can do several different jobs and creating a wash using it is very straightforward. If you are using Vallejo paints then I would recommend their own Glaze Medium. Simply put the paint on the palette and add a couple of drops of the glaze medium- ensure you have a moistened brush and mix it up until you have something with the consistency of a wash. Then simply apply as you would a wash- although just be careful not to let it sit too long in areas you do not want the wash to touch- just keep your brush moist and move the paint around as desired. You don't need to add thinner to it but you can add water to it if you want to thin it a little further.

I would recommend using a wet palette.