I put the spices into the vodka at the same time that I brew -- that way the infused vodka will be ready when I bottle. Just a teaspoon or so of spice for 1/2 to 1 small spice bottle of vodka. But the amount is going to depend on how strong your spices are. Let them soak at least a couple of weeks, preferably longer, and shake the bottle from time to time to mix things up.
Then add a drop at a time of the infused vodka to a glass of your beer, just before bottling. Once you find an amount you think tastes best, scale that up to your batch size, and add that to the beer. Most likely, it will still be a very small amount of infused vodka.
Since you're wanting a blend, you might either do the spices separately or smell check your vodka as it gets flavored to see if the balance needs adjusting. I have some mulling spices infusing that I'm doing that with.