From TSR post

You can catch only one Smeargle per day.

You can find Smeargle whether you are using AR+ or regular AR.

Smeargle only photo-bombs the first picture you take per session.

    When he photo-bombs you, a copy pic is made. Because of this, its possible he shows up on what would seem as the second picture.

    You do not have to switch Pokemon each session, you just have to start a new session (close camera, select Pokemon again, etc).

    Smeargle takes the moves of the Pokemon he photo-bombs. This includes the moves of other Smeargle.

    It APPEARS Smeargle can copy some community day moves, but not all. It’s possible he only has a chance at copying them. If the copy fails, he gets a random move.

    Using a tm on a Smeargle will give it a totally random new move. This feature will soon be removed though, apparently it was not intended.

    So far, shiny Smeargle is not in the game.

    Smeargle’s moves do not receive a weather boost.

    You can block Pokemon Go from accessing your gallery to make it so you dont end up with tons of saved photos. This doesn't stop Smeargle from being able to appear.

    Smeargle will appear next to you on your map after he’s photo-bombed a pic. Simply close the camera to see him spawn. CAREFUL, he can run.

    When catching Smeargle, make sure to be STANDING STILL, and make sure your connection is good. Smeargle spawns right where you were the moment the photo-bomb happened. So if you are in a vehicle, you wont catch him, and bad connection can mess this up too. KEEP THIS IN MIND!!!

    In my own experience, it takes a second for him to spawn, where he appears as a white light first for a moment.

    When you take a photo, a ring pulses around it in the bottom right corner. If it pulses 3 times, instead of the normal 1, that means the photo was photo-bombed.