There are lots of posts on this in the forum. If you search for nucleus, you will likely find them. However, let me outline it for you.
- You build your hive. It has frames but no bees.
- You order your nucleus. It will likely be 3-5 frames of bees, honey and a queen. Most nuclei in the US are made on “Langstroth deep frames” which is what is in the Flow hive lower box (brood box).
- Let’s assume you get a 5-frame nuc. Now you remove 5 frames from the middle of your lower box in your 8-frame Flow hive - OK, it is asymmetric, so you will have one frame next to one wall, and 2 next to the other.
- When your nuc arrives, you very gently take the frames out of the nuc box and put them into your Flow brood box in the same order and orientation that they were in the nuc box.
It sounds hard, but it really isn’t. You can do this! 
Dawn