The Camera Upload feature in Plex for Windows Phone was designed to just work (after enabling it of course). If you find that it doesn't work "automagically", please read through the following:
Open up the app and select settings (in the appbar) -> slide to camera upload. On that screen, is the feature enabled? And have you configured the library and album settings?
If so, then at the bottom it should say "next upload - photos taken since X", where X is the last time the app uploaded photos to the server.
Next please go select status (in the appbar) -> slide to camera upload. If the background task isn't working correctly, you'll likely see a bunch of images there that haven't been uploaded. Try tapping the upload now button in the appbar. That should upload all the images immediately. If that doesn't happen, and you're confident your Plex Media Server is contactable, then please post a new topic in this forum.
The most likely reason for the application not uploading in the background is that the Windows Phone OS isn't running our upload background task on schedule.
First, determine which OS version you have by going to the system's settings -> about, then reading the Software line.
Open up Battery Saver -> Scroll to Plex, then make sure that Allow app to run in the background is on.
Go to Settings -> slide to applications -> background tasks -> scroll to Plex and make sure it is allowed.
The task may be off if you have too many other tasks running in the background. After enabling it, be sure to open the Plex app again so it can get back on track.