Flow super on top above a queen excluder. You can rub wax or spray with sugar water to encourage them to use the flow frames. If it is a flow super hybrid you could also move one or two frames up with out the queen. If it is a full flow super not so easy.
When you put the super on make sure the hive is bursting at the seams with bees and brood. This will give them no option but to use the flow super to store nectar.
Have a look at the FAQs of flow and you may also glean some other gems by searching the forum.
I should have been more specific. I am in Maryland, and here everyone reccomends to full brood boxes before honey super. So i will be adding 2nd brood box first
Put it on the top of your present brood box, when that is 80% full of honey and brood fit a metal queen excluder then the honey super. The plastic queen excluders are more of a barrier to bees and tend to become brittle with age. The metal wire excluders seem more acceptable to the bees, need less cleaning too.
I am guessing you have a cold winter so the two brood boxes so the colony is strong over winter to keep the brood warm.
Regards
You can add the new box above or below the existing one. I only add below if the existing box is a little low in numbers, and not particularly full, but I am going to be away from the apiary for a couple of weeks. If the existing box is strong, it is much easier to add it on top.
I wouldn’t. The less you move frames around, the better for the bees. When you move stuff, it is like somebody coming into your house and changing the position of the kitchen and bedrooms, etc. Very disruptive!
I don’t know what kind of entrance reducer you have, so it is hard to say. If the bees are really crowded, trying to get into the hive, they probably need a bigger entrance. My hives are currently 3 boxes tall (2 brood deeps and a super), and the entrance is about 6 inches (15cm) wide. I never have it bigger than that, as good quality bee research has shown that to be the size that bees prefer when given a choice. My bees are perfectly happy with that size, and it means that they have less trouble defending the hive from pests and robbers.
You should be having a huge flow in Maryland right now: Just add it to the top and the bees will do the rest: Keep an eye on it because a strong colony on a strong flow can fill it in a week or two.