Placing the chocolate while his back is turned with 〜間に

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

Urushi prepared chocolate to give Ayumu for Valentine’s Day but is too nervous to give it to him. After the two play shogi during their club, Ayumu turns away from Urushi as he puts the shogi table away.

うるし:: 「今(いま)。歩(あゆむ)が　後(うし)ろ向(む)いてる間(あいだ)にかばんに入(い)れればいいじゃん！！」; “Now! While Ayumu's back is turned, I can put it into his satchel!”; Literal: “Now. It would be fine if I put it in his bag during the time Ayumu is facing away!”

Key Points

1後(うし)ろ向(む)いてる間(あいだ)に marks the opening

〜間(あいだ)に means that one action happens during the span of another action or state.
Urushi wants to put the chocolate in his bag during the time Ayumu is facing away.

2向(む)いてる is a casual contraction

向(む)いてる is shortened from 向(む)いている.
It describes Ayumu’s ongoing state: his back is turned right now.

3入(い)れればいいじゃん suggests an easy solution

〜ればいい means “it would be good if…” or “all I have to do is…”.
The casual じゃん adds the feeling that Urushi has just realized the obvious move.