Low poly model.
High poly model.
Make sure both objects are in the same location. Select the low poly object and create a material. Create an Image Texture node and Normal Map node. Plug the colour from the texture node to the normal map node. Plug the normal into any shader, in this case it's the default Principled BSDF. Use the image texture node to create a new image of any size. Set the texture node Colour Space to Non-Colour.
Now select both objects, having the low poly object with the material attached as the main selection.
Go to the Render Properties panel > Bake. Change Bake Type to Normal. Check the Selected to Active option. The Extrusion slider will need to be adjusted depended the scale of your object. Click Bake.