First you need to catch at least 10 different pokemon (if you catch 1 and evolve it, this will count as 2). Next you need to use cut after Diglett's cave. On route 2, there is a small house with one of Prof. Oak's aides inside. Talk to him, and he'll give you HM with flash in it. Teach this to a pokemon and you'll be able to use flash.
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The place where it flashes is where you can find the Pokemon It would flash blue if it was found in the water
Only the electric types. Like Abra and the ters its good for
Get a Pokemon that can learn flash and put them in your team , teach them flash and go through following the ladders like Mt Moon
Get Flash from a guy a route 2 Get Flash from a guy on route 2
Flash, I believe.