These three how-tos all solve the following scenario, and have a shared set of steps to load in the two Levels you are going to work with. Follow these steps, then select the how-to you want to continue with.
Level Streaming Scenario
Starting with the main level in the Sun Temple project, we have split the level up into the interior space and the end patio with pillars overlooking the ocean. In the wireframe view below, the teal wireframe shows the persistent interior level, and the yellow wireframe shows the patio level that is going to be streamed in. The sky and ocean are in the persistent level, as there are a few windows in the main temple that allow the sky and exterior to be seen.

In the interior of the temple, there is a bend in the hallway that hides the patio area from view.

We want to start streaming in the patio level here, so that by the time the player rounds the corner and begins approaching the patio, the streaming level will be loaded and visible.

As part of the setup, we have two levels, SunTemple_Persistent and SunTemple_Streaming. Our Player Start is in SunTemple_Persistent, and our player in the game is represented by a Character.
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Open SunTemple_Persistent in the Content Browser.
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Move the Player Start to the very beginning of the temple.
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Click on Windows, then select Levels.
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Click on the Levels dropdown menu, then select Add Existing... to add a new sublevel.
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Select SunTemple_Streaming to add in the Open Level dialog, then click on Open.
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Right-click on Persistent Level and select Make Current from the dropdown menu.