Theres also a special machinist technique of bluing then scribing to port motch both the head and the mani. to the gasket size. If You have no acces to such technique Just place the gasket and bolt it or tape it in. Then Scribe it (with a sribe lol) Around the ports Untill its matched with your gasket. You can then use a dremel (as stated before) to CAREFULLY remove the material.
Try not to change the shape of the port or get to ambitous and take away to much material because this can either make you port job ineffective or Even lose power.