The material components of Warding Bond are (emphasis mine)
*- (a pair of platinum rings worth at least 50 gp each, which you and the target must wear for the duration)
Since the description clearly states that the two creatures must wear such rings for the duration, and the duration begins whenever the spell's effects begin, is not clear to me if during the casting the cleric\$^\star\$ and the target must already wear the rings.
The only aspect that I am sure of is that the cleric\$^\star\$ must have (in some way, on its finger or holding in the hand) the ring at casting time, per material components rules. It is true that during the casting they may put on the ring, hence satisfying the requirement. For what regards the target, must they wear the ring at casting time?
This could be not a problem outside combat, but during a fight the action economy may draw some attention to this aspect of the spell.
Suppose that Eddy the Artificer is fighting alongside Adrian the Fighter, who is severely injured but has to pass through several enemies in the battlefield (receiving hence several opportunity attacks) for reaching with the Dash action the lever that opens the drawbridge and make enter the reinforcements. Eddy starts to cast Warding Bond, but Adrian is not wearing the ring Eddy gave to him, because it is too fancy&showy. Assuming that Adrian can easily take the ring from its pocket, how they can resolve the situation for succesfully cast Warding Bond? Do they both need to wear the rings at casting time?
\$^\star\$ Or the paladin or the artificer or any caster that gains access to this spell.