https://support.riotgames.com/hc/en-us/articles/208754686--Remake-FAQ
/Remake becomes available at 3:00 if a player is disconnected or flagged as inactive for 90 seconds. First blood against a player's team before they leave/dc will prevent a /remake vote.
At 3:00, the impacted team has 60 seconds to start a vote by typing /remake in chat. Once the vote begins, you have 30 seconds to vote yes or no.
In a 4v5, at least two players (30% of the team) need to vote yes for the game to end. In a 3v5 or lower, you only need one player to make that call.
Once the vote passes, the game ends. You'll neither gain nor lose LP/BE/XP, and you won't see a win or loss on your record. The inactive players (and Diamond IV and above players in their premade) are punished.