Far easier then you would think, so bring down that anxiety level.
Once you unlock the first stage of Gunmetal Creek, you also open up the Skatepark - Grind Switch level. Head on in there, and the game provides a tutorial on how to do a grind switch. Jump on the grind bar provided, and keep jumping and grinding doing perfect grinds and switching them throughout the ENTIRE level.
I finished with a 48x combo and well over a million points. Just stick the landing at the end, but this is the easiest, earliest spot to pull off a million+ combo.