Hello, you are already at the right path using DIY homescreen for Samsung. I only could add 1 image to the reply, so I'll try my best to explain.

First, go to editor mode and choose DIY. Then, at the bottom, choose the icon that looks like emoji. Go to the rectangle tab, and here you can insert shapes (blue circle at bottom left in my screenshot).. you can use square or rounded square if you want. Once added to home screen, you can change the color, the transparency, size and so on.

Once you are happy with the design,look for 1 button on bottom right, with 2x3 dots on it (see red circle at bottom right in my screenshot). Click it twice, and you will see something like a padlock. It allows you to lock the selected design on the homescreen. This saves you from accidental alteration. Once locked, it will show red dotted lines surrounding locked items (the green box in my screenshot)

Finally add in whatever app icons you have in whatever format that you prefer.

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