If you've customised a shield in their inventory from the character sheet, they should be able to just select it in their inventory and equip it from there
If you've made a completely custom item in the sheet, those can't be equiped
If you've made a homebrew item, the character sheet will need homebrew content enabled and then you'll need to add it to their inventory the same way you'd add any other item
I've made a customised shield for one of my players... Is there a way I can make it so he can equip/unequip it in D&D Beyond?
If you've customised a shield in their inventory from the character sheet, they should be able to just select it in their inventory and equip it from there
If you've made a completely custom item in the sheet, those can't be equiped
If you've made a homebrew item, the character sheet will need homebrew content enabled and then you'll need to add it to their inventory the same way you'd add any other item
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