First thing to determine is tye of decal you're using...waterslide or dry transfer...maybe the instructions mention something, or someone else has built the 1/700 Tamiya Yamato.
Here's my suggestion for dry transfer...rub one side of the flag onto a piece of aluminum foil...then trim the foil to the decal...then add the second transfer to the opposite side. I'd give the whole thing a coat of clear once I was done.
For waterslide, simply do the same thing...aluminum foil allows the decal to adhere to something other than itself, and also allows you to 'wave' or curl the flag.
Regards,
Jeff Herne