Before every match you and your opponent chat message someone, who is in neither you nor your opponent's element, 3 cards.
Are we required to play our matches like a spontaneous speed building on chat ? Or can we arrange our bans with our opponent 1-2 days earlier and have time to build related decks ?
Most people I expect to do the banning and playing pretty close together. if you and your opponent(s) arrange to do the bans together then come back later and play the match that is also fine as long as that is mutually agreeable.
You still need to do the bans live in chat together through a ban arbitrator per the rules. The rules do not specify a time after the bans are determined in which the match must be played (other than the main Phase 2 timer)