Create a Social Media Header

For this project, I used Paint.Net again. 
Open the program, and edit the canvas size to: width 828 and height: 315. 


Color the background to black. Then go to Layers > Add New Layer.

Now, you should already have the covers for the series, including the upcoming book's cover, preferably, the final version.
So go to File > New and open up all cover arts for the previous books and the latest one. In this case, I used 3 book covers. 
They should all be in the same sizes (my images were 1600 in width and 2400 height). Next, you want to resize the images to shrink them. So go to Image > Resize and at "By Percentage, change the measurements down to 8%. Resize all used images, then copy and paste them one-by-one to the wide, rectangle background, into different layers. Drag and drop each to your liking with the latest book's cover being on top, then merge them into one layer, having only 2 layers in total, the background and the book covers:

Now, let's create the "glow": select the second layer, then go to Layers > Duplicate Layer.

Then select the top layer - the duplicate - and go to Adjustments > Black and White. Then Adjustments > Brightness and Contrast and set both brightness and contrast to minimum, moving the handle to the left. your selected layer's image should turn black. Now go to Adjustments > Invert Colors, then Adjustments > Brightness and Contrast and move both handles to the right. Your image should be white.
To make it glow go to Effects > Blur > Gaussian Blur and drag the handle or write in 24 to the little square. 
It now should glow.
Go to Layers > Move Layer Down. 
The glow layer should now be underneath the book covers.

Now, all you have to do is to add a new layer and write the text. Include your upcoming book's title, the series' name (if it is a series) and a release date or say it is "Coming Soon" or something like that, as you see on the top image. Include your website or email sign up link.

You can play with colors and add a frame.

To create a blend-color background, select the bottom layer, that's now black. then go to Colors and select a dark primary and a lighter secondary color. Then go to Tools > Gradient and on the toolbar, pick the first one "Linear". Now click the upper right corner of the image with left mouse button then click again and leave the button down and drag towards the bottom right corner of the image.

To add the frame, add a new layer and go to Tools > Shapes. Select rectangle, set shape draw/fill mode to draw shape outline. Brush size can be any, I picked 3 and then I picked a color (I picked yellow for a contrast) then drew a frame.

You can change colors under Adjustments. Or recolor layers to your liking.

Save as paint.net - so you can edit it later and save it in jpeg or png and flatten image to use it as a header.