What site do I use to make before/after split screen photos??

Hey, guys!! Can ya help a girl out?? I am wanting to make the split screen (side by side) before and after photos and I don't know where or how to do so? Thanks

I use an app on my phone called FrameFree.

I just used paint honestly.  I never thought to look for a site or app to do it.  The pictures may come out better if I did though!

I use an app called Pic Stitch.