In the town brightbulb. theres a tower on the east end where there is a sailor is standing who won't let you go until you steal a pokemon from the one guy. go inside and beat the trainers to the top and bam Flash is yours
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how do you use flash? just go to pokemon and go to the pokemon that knows flash. press A and it will say in blue text "Flash" select that and it will use Flash. it's very useful in dark caves.
When a Pokemon has the move Flash, you go into Party, then select the Pokemon with Flash using the 'A' button and there should be an option that says "Flash". It is only usable in dark locations or within battle.
Teach one of your pokemon 'flash' using a TM/HM
Use the HM Flash this is given by the sage at the top of sprout tower.
In Pokemon Platinum, Flash is a TM.