if you make a mistake with one step, it will end up looking bad
scarves are cute but if you mess up one step, it will end up looking bad
「一歩」＝ one step
「間違える」＝ make a mistake / mess up
「ダサい」＝ uncool / unattractive / not cute
「~なりかねない」＝ turn out (badly) / end up (looking bad)
「一歩間違えるとダサくなりかねない」＝ if you mess up one step it might turn out badly / if you make a mistake with one step, it will end up looking bad
「スカーフってかわいいけど、一歩間違えるとダサくなりかねない」＝ scarves are cute but if you mess up one step, it will end up looking bad / scarves are pretty, but if you make a mistake with one step, it will end up looking unattractive
イギリス 
