By possessing it using the Possess node.
But if you’re asking how to convert a mesh that you placed into your scene to a blueprint, you can simply click on that button with a branch icon on it next to the compile button on the top left while the mesh is selected, and click on the option to convert it to a blueprint.
First, make sure it’s Mobility setting is set to Movable. If so, you can simply use the Add Movement Input node and trigger it with your movement input action, just like how you do with your first person character.
Hope these help! 