I'm going to forming up a slab with two steps down. Because of the long run (8m long) my question is which is the best way to support these staps, and at the same time make them pretty straight. Thanks

The steps should be supported on undisturbed earth. Build your forms and tie it into the slab, this is where you need to take care to make sure they are straight. 7" maximum rise on the steps is code here, not sure where you are.