Apply group policies to more than one forest/domain

Hi,

I have 3 forests. One is a single-domain forest, the other two (which are identical in structure) have 3 child domains.

In the first forest (single domain forest), I have set some group policies. I would like to apply this to every domain in my other 2 forests. 

Is there a quick way to do this that is "tried and tested"? The objective is to not repeat what I do.

Thanks