You can use your Gacha Sticker Vouchers by opening the Menu, then going to [Exchange] → [Gacha Sticker Exchange].

Gacha Sticker Vouchers can be traded for any characters other than limited or Colorful Festival-exclusive ones, while Limited Gacha Sticker Vouchers can be traded for any characters that were featured in limited or Colorful Festival-exclusive gacha.

One Gacha Sticker Voucher is equivalent to 10 Gacha Stickers. On the inverse, you can trade 10 Gacha Stickers for one Gacha Sticker Voucher.

However, please keep in mind that you can only use up to 10 Gacha Sticker Vouchers when trading for a particular character.