You get Hm flash from the hiker in granite cave in dewford.BEWARE!You can only use flash if you get the knuckle badge from Brawly
Steven will give it to you
You go to the cave in dewford townand its the first Hiker in there.
talk with steven in the MOSSDEEP
There is a house in Rustboro with a man who will give you the HM if you talk to him.
all the way back in butts cave
use flash
by going to lilycove and talking to the move deleter
You need to get the hm cut then teach it to a grass Pokemon
Wally's dad gives it to you after getting the 5th badge.
y beat brock at pewter Jim
teach a Pokemon the hm move flash
flash isn't a HM it is a TM