As per HQ2 p14.
The flaw is a resistance against an action of the character to be overcome to proceed with the intended action or a modifier (value/5) at an action.
In the first case you, the character, must have an ability to roll and the flaw is the resistance. if you don't have any ability try to stretch some ability or keyword, if not roll with a standard level of 6. In the latter case, that means that you are prone to the flaw.
This would d be annoying in the game so as Gamemaster you can suggest to the player to take some ability as "strong-willed" or similar.
In the second case you simply subtract the 0.2 x flaw from the ability you roll meaning that the flaw is hindering you so reducing the skill value.
Some example:
if you have the flaw gluttony, any roll that requires attention in a place with delicious smells will be hindered by the loss of concentration.
If you have gluttony and some Spymaster will offer you a very rare bottle of wine for its own purpose you must overcome the flaw in order to refuse. In that case, you must use a suitable ability depending on the story.