Please go up the stairs and line up on the right hand side at the rear.
Please go up the stairs (behind the line that is already there)and make a line on the right hand side.
「階段を上がって右側後方に並んで下さい」= Please go up the stairs and line up on the right hand side at the rear. / Please go up the stairs(behind the line that is already there)and make a line on the right hand side.
ボキャブラリー
go up the stairs = 階段を上る
line up/make a line = 並ぶ、行列を作る
on the right hand side = 右側
at the rear = 後方
behind the line that is already there = もう直ぐ出来ている列の後ろ
Please go up the stairs and line up towards the rear on the right.
「階段を上がって右側後方に並んで下さい」という日本語の表現は、英語では "Please go up the stairs and line up towards the rear on the right."と表現できます。ここで、"go up"は「上る」、"stairs"は「階段」、「line up」は「並ぶ」、「towards the rear on the right」は「右側の後方に向かって」といった意味です。