Log into the OneDrive app and allow it to access your photos. Hit the icon that looks like a person and tap the gear there. Look for "Camera Upload" and set that to On.
The + icon next to the Search allows for you to upload. Just tap "choose existing" and select what you want to upload from there. If you want it to work immediately, just tap photos when you open the app and then you'll see if your camera uploads are "up to date" or it would tell you how many are left to upload.