Okay. So the easiest thing is to click the model glTF (or whatever format) in the file browser in Godot, then click the Import tab, select Gen-Lightmaps and click Reimport. The lightmap scale should be good, if you did not scale the model in Blender. It is a good idea to select the model in Blender before you export and do Apply -> All Transforms, to make sure there is no weird scaling.
Then, in Godot, click the MeshInstance (inside the imported scene) and double check it has a correct UV2 map. When you click the mesh, a Mesh button will appear on the top of the viewport, click that and choose View UV2. If it is missing, or looks strange, click Generate UV2 for Lightmap. Then on the right on the Inspector, click the Mesh and look at the UV dimensions. The default is usually fine, but you can change the number manually if the quality is not good. You can also set it to 0, 0 and then customize the default on the Baked Lightmap node. Then click on the Geometry section and make sure Use in Baked Light is selected.
If you have multiple objects that share a Mesh, you need to duplicate that object scene, and then click the Mesh, and choose Make Unique. In the main scene, make sure you have a Baked Lightmap node and that the extents covers the entire area you want to be baked (you can leave an extra buffer around the edge to be sure). The default medium setting should be fine for testing. Click all lights in the scene, and choose Bake Mode all. You also want to enable shadows (if you want shadows). After all that you can click the Bake button.
