"What is the best way to cut a 30mm tyre to 28mm or 26mm??"
You need to put the tire on a truer. I have seen some guys use an Xacto to cut the tire, just holding it as the tire rotates. What might be a bit easier would be to use a fine tooth hacksaw blade, (like 32 teeth per inch). All you really need to do is hold the tool on the tire as it rotates. And please , make sure it's rotating AWAY from you, and you have a backstop, especially if you go the Xacto route.