need help with replacement case artwork

[noiseredux]

So I have some loose DS/3DS games which I picked up super cheap, but it really bugs me to have such tiny carts out of cases. Some of the games aren't on The Cover Project, so I could use some help walking me through how to make my own replacement art. I think finding a blank DS/3DS cover template is easy enough. But I don't really have the skills/knowledge to go from there. Also, I use a Chromebook at home (with support for Android apps). Can anyone suggest an easy app capable of easily copy/pasting cover art onto a template for printing? And maybe walk me through the whole template thing (IE: layers or whatever for keeping the DS/3DS banners on the cover)?