Choose a stage or goal from the singleplayer/arcade time attack menu.
Stage-based times: only ghosts that go through the "Retry" checkpoint will be uploaded, after reaching a satisfactory time, go through the course again and head to "Exit".
Goal-based times: after reaching the goal & setting a new time, you'll be prompted to upload your ghost.
To submit a score:
Log into the server.
Play a score-based singleplayer/arcade mode.
If you beat your previous score, your new score will be uploaded to the server.