If you're asking how to move from tomato to dd-wrt directly, I'm not sure you can (maybe you can, but I didn't check). Usually there are instructions for moving back to the original firmware of the manufacturer. And at that point you can just install dd-wrt as normal.
Yeah, itâ€™s a bit of a pain if you donâ€™t really have to, put locating information about moving from tomato straight to dd-wrt always seems sketchy to me.
At least that's been the approach I've used in the past, just to be safe.
Same here I always go back to the manufacturer firmware first. This reduce your chances of bricking your router.