To keep the bottom as bushy as possible, you need to keep the top of the hedge narrower than the bottom, so that as much light gets to the bottom of the plants as possible.

In other words, if you are standing at one end of the hedge looking at it in profile, the shape of the hedge should be somewhat like an "A" (not as exaggeratedly narrow at the top as an "A", moderately so.) Many hedges end up looking like an upside down "A", which ends up shading the bottom of the plants too much, making it go bare.

Light, frequent shearing gives bushiest results. Over the long term, a harder shear every few years can stimulate a larger amount of fresh growth.

Wait until you see a little new growth before you start shearing. When you see some new growth, you'll know the plant is established enough to take a light trim.

Thank you so much @hoovb for your answer. This is my first hedge and I really appreciate your advice. Yes, I've read about the importance of a trapezoid shape and planning to maintain it. I do see some new growth already, even though it's been only a week, but I'll postpone first pruning till, let's say, early spring? What I've done so far, though, was trim:

1. really long branches by about 1/3, and
2. low hanging branches that were laying on the ground

I hope it's OK, since it's only been a week after planting FN. Also, I am not sure what to do about the top of each tree. Should I let them all grow to its final height ( ~9 ft.) or is it OK to start trimming it sooner? If I cut the top off the main stem, will it stop growing?

What you did sounds fine; they are tough plants. If you cut the top off the main stem, it will branch at growth points below the cut. You will get a bushier top if you cut somewhat lower than your desired height and let multiple new shoots grow up to the desired height.

Always remember that In nature, this is a plant that wants to be a large tree, so they will
keep trying to grow. Long term, It will be a battle between you and the
tree's DNA. You can win, for a while.

One more question, please. When I do my first thin-out trimming in about a month or two (when I see new growth), is it OK to cut more than 1/3 from each branch in the top section, since this is where the most foliage currently is? My current goal is to promote growth at the lower portion of the trees, so i was thinking of doing this:

1. Prune about 1/2 of each branch in the top section of the tree that is currently the bushiest in order to route more nutrients towards the lower section.

2. Prune just a little (about 1/5?) off of each branch in the bottom section of the tree where it's currently the least bushy to promote new growth.

3. Let the main stem grow until it reaches the desired hedge height (8.5'-9') and then cut it. Currently most of the tree tops are already at 7' - 7.5'.

4. Maintain trapezoid shape with each pruning session

Please let me know if this makes sense for the first pruning session. I want to make sure I am not too aggressive on these new trees. Thank you.