This is amazingly simple! I use scrapbook paper as an insert in a clip on clear iPhone case!
I found a clear iPhone case at target for about $3 on sale. You can get them on eBay too for just as inexpensively. I have also seen them for older models at the DOLLAR STORE!
When I bought my case it had a paper iPhone inserted into it, so I took that and took and exacto knife and cut out the camera hole.
Then I traced that onto the back of my pretty scrapbook paper. You can also easily trace your phone or the case to get the camera slot right. Then just slip the paper in and click the phone on top and you're good it go! I change mine about once a week sometimes more sometimes less but it keeps it fun and lets me control what I looks like.
I have so much scrapbook paper and no longer scrapbook, so it's nice to have a little project to put some of it to work. I have thought about using glitter paper but I worry about loose pieces getting stuck in all the nooks and ports of my beloved white iPhone, so we'll see.