If you lift up the corners of your mouth and make a smile, your brain will think that you are having a good time, and as a result, you actually will end up feeling good.
ご質問ありがとうございます。
"If you lift up the corners of your mouth"＝「口角を持ち上げて」
"and make a smile,"＝「笑顔を作ると」
"your brain will think that you are having a good time,"＝「あなたの脳はあなたが楽しい時間を過ごしていると思い」
"and as a result, you actually will end up feeling good."＝「実際に楽しい気持ちにあなたはなります。」
ご参考に。
アメリカ合衆国 
