To start multitasking, place your finger on the bottom of the screen, swipe up and hold your finger in place.
"画面下から真ん中まで指でなぞってマルチタスクを表示して" という日本語は、以下のように英語で説明ができます。
「To start multitasking, place your finger on the bottom of the screen, swipe up and hold your finger in place.」
To explain in more detail you can say
「To start multitasking on an iPhone X, place your finger on the bottom of the screen, swipe up and hold your finger in place until the multitasking screen appears. 」
I hope this will help you.
お役に立てば嬉しいです
イギリス 
