This forum post assumes you already know how to upload aircraft with custom thumbnails.
This forum post is with regards to this mod and will go through the basics of how to use and manipulate the various greenscreen features of the mod in order to get the perfect thumbnail.
Please refer to the GiF files attached in this post, as they can explain everything a hundred times better than words can.
I will use various successful builds as examples. You may click on the URL next to the pictures to view the aircraft for yourself.

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Chapter One:

How to Take Appealing Screenshots

1) Ensure that your aircraft is the center of attraction. You may use blinding red text and obnoxious arrows, however, bear in mind that your AIRCRAFT is the product you are advertising, and the same must be clearly and distinctly visible.

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2) Try adjusting the FoV (Field of View) of the aircraft in order to get the best angle using which you can take the picture.

HOW CAN I CHANGE THE FoV?

a. Press the ~ (tilde) key on your keyboard, also called the ` (backtick) key. (below Esc and above Tab).
b. Paste the below text in the console (the textbox that appears).
>>CameraManagerScript..SetCameraFov 30
Replace 30 with whatever FoV you want. The default amount is 60.

Credits: MisterT's Supermarine Spitfire

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3) Use the day-night cycle to your advantage. People generally consider the "golden hour" (16:00) as the best time for the same.
You can adjust the time of the day as follows:
Settings --> Game Settings --> Adjust the slider accordingly

Credits: ALRX's X-17 Trainer

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Chapter Two:

Using the Greenscreen Mod

1) Well, firstly, download the Green Screen Mod

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2) The Green Screen Mod is a custom map, so, load it in using the Mod Sandbox Maps option.

Select the correct screen option (Green, Blue, Red Black, White).

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3) Use the tips given in Chapter One to find the perfect angle and take the screenshots.

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4) Now this is tricky, so pay attention. You need to edit the background out of the screenshots.

You can use most photo editing applications for this, but I personally favour pixlr.com/x/ as it's easy to use and perfect for our purposes.

In pixlr.com, upload your screenshot (it's pretty intuitive from the UI), and it'll open the editor.
In the editor, search for the cutout option (icon is a pair of scissors) on the left-hand side. Click on that, then click on Magic Cutout. Click on the background and it'll remove the background. Repeat as many times as necessary to remove the background, but it won't work for very tiny details. Those can be removed with the brush tool or the shape tool in the cutout tab.

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5) Add a background, text and all the rest!

Now, search up a suitable background. Load it into the editor. Mess around for a bit, add some text, and bingo! You're all set!

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Final Result:

Beautiful, isn't it? Credits to the plane go to DestroyerZ117.

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Chapter Three:

ReShade

If you're looking to be really fancy with Ray Tracing and all that, here's a cool guide by WEAPONSMITH aka. NumbersNumbersTheMan aka. 7825.

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If there's anything I've missed out, please do let me know. Thank you for your time.