You can use Gacha Sticker Vouchers by going to the Menu > Exchange > Gacha Sticker Exchange.
Gacha Sticker Vouchers can be traded for any Character other than Limited/Colorful Festival Exclusive Characters.
Limited Gacha Sticker Vouchers can be traded for any Character that in appeared in Limited/Colorful Festival Exclusive Gacha, including Limited Time/Colorful Festival Exclusive Characters.
1 Gacha Sticker Voucher is equivalent to 10 Gacha Stickers.
You can trade 10 Gacha Stickers for 1 Gacha Sticker Voucher.
Please be aware that you can only use up to 10 Gacha Sticker Vouchers when trading for a Character.