Make sure you don't mix up two things that are not directly related.
Never get confused between two separate things.
"Make sure you don't mix up two irrelevant things."＝「関係のない二つの事を混ぜて考えない様に徹底して。」
Make sure = 徹底する、必ず～する様にする
mix up = 混ぜる（混ぜて考える）
irrelevant ＝ 関係がない
"Make sure you don't mix up two things that are not directly related." = 「直接関係のない二つの事を混ぜて考えない様に徹底して。」
directly related = 直接関係がある
"Never get confused between two separate things."=「絶対に二つの別々のものを混乱して考えないで」
Never = 絶対に～しないで
get confused = 混乱する、間違える
separate things = 別々のもの
"Think logically, how are A and B related? How would B change is A changes? They are not related at all, right? (They are completely irrelevant, right?) So you don't need to think about B when you are thinking about A. You will only confuse the other person if you start talking about A when you're talking about B. Make sure you don't mix up two irrelevant things."
日本 
