Combining Meshes?

I know that combining meshes is possible, I'm not a noob programmer. But I need to combine objects meshes into one instance with one mesh at runtime. And keep their shared material. I simply cannot get my head around how to use Mesh.CombineMeshes It just confuses me so much, And I don't have time to do 17 hours of trial and error a day to figure it out. I'm going to be going to college in 2 days. **So how could I use Mesh.CombineMeshes when instantiating a simple clone of an orginal object and combining the two meshes together to form one object instead of 2?**As doing this repeatedly without merging will cause extreme lag.