I use: https://www.canva.com (and type in the custom dimension) to assemble everything, I use their backgrounds sometimes and fonts as well!
To “cut the characters out” I use: https://www.remove.bg (usually work perfectly! and gets those white lines)
This is the simplest method I have learned for amazing looking backgrounds, covers, & splashes! If you want more help, PM me or visit my shop! 
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