Well. It really depends on how accurate you want to simulate the motion of air passing over the cloth. To do a full simulation would be kind of complicated. I figure you would settle for something that looks half decent?
You should look to using sine waves. However, I'm really not sure what would be the best way to go about doing this in BlitzMax. You could, most simply, chop your flag into lots of verticle strips, and then move each strip vertically, individually, with the sine wave moving from left to right (ie use something like sin(angle)*30 or whatever). If the movement isn't too extreme it will look okay.
You should look to using sine waves. However, I'm really not sure what would be the best way to go about doing this in BlitzMax. You could, most simply, chop your flag into lots of verticle strips, and then move each strip vertically, individually, with the sine wave moving from left to right (ie use something like sin(angle)*30 or whatever). If the movement isn't too extreme it will look okay.