Make sure to tie the top of the bag after putting everything inside.
"レジ袋に注文されたものを全部入れたら上を縛って"
->Make sure to tie the top of the bag after putting everything inside.
「お客様から注文を受けた物が全部そろったら、レジ袋に全て入れて上を(ほどきやすい縛り方、リボン結びで)縛って」
When you have gathered all of the ordered items, put them all in the bag, and tie the top of the bag (in a way that's easy to untie).