How can I measure / weigh really small quantities?
Simplest solution: Buy a more sensitive scale. There's plenty around that can measure grams.

If that's not an option, you can sort of just about get it quite, but not completely wrong by using measuring spoons:

A full teaspoon with something in it is usually around 5 grams. 
A quarter teaspoon would be 1.25 grams, if you happen to have a 1/8th measuring spoon, it would be around 0,6 grams.

I'm not very proficient in baking (or not at all, really), but what I notice (or think I noticed) is that correct measurements in baking are more important than they are in cooking. So the best advice would still be (since you're baking a focaccia) to buy a more sensitive scale.