Keeping still while her ribbon dries with 〜間

Grammar Explanation: During the same time with 〜間(に)

Kyouko’s ribbon needs to be washed. Since she is a jiāngshī (vampire), she will uncontrollably attack people to drink their blood when her ribbon is removed. For this reason, her brother tries to tie her up with a rope. Kyouko hides in the bathroom, then sneaks out the window only to find Kenji waiting outside.

ケンジ:: 「干(ほ)してる間(あいだ)じっとしてればすぐ済(す)むだろ。」; “While it's drying, if you'd just keep still it'd be over soon.”
ケンジ:: 「なんで毎回(まいかい)そんなにイヤがる！！」; “Why do you seem to hate it so much every time?!”

Key Points

干してる間 means while it is drying
- 干(ほ)してる is the casual form of 干(ほ)している.
- 間(あいだ) marks the time span during which the ribbon is drying.
The condition is じっとしてれば
- じっとしていれば means “if you stay still”.
- してれば is a casual contraction of していれば.
すぐ済むだろ minimizes the problem
- 済(す)む means the matter will be finished.
- だろ frames it as obvious: if Kyouko waits, it will be over soon.