Assuming your level is at least 4 times as big as your viewing distance, you can use the following technique.
- Split your level in at least 4 pieces*
- Loop the position of your player / camera at the level' dimensions.
- When a level piece is more then half the level size away, move it a "level size" closer ( per axis ).
So for example .. the red player starts moving towards a edge of the level.
Now that he's gotten closer to the edge of the level then the center, the level pieces jump to the other side.
After passing through the level boundary ( green rectangle ) the player ends up on the other side, which makes the level pieces move back into place.
*It is recommended to make each piece slightly bigger then your viewing distance, this will allow you to do some very cheap & efficient clipping.