To redeem your Steam Wallet voucher, please log in to your Steam account. Then, select the "Manage Account" menu in the top right corner of the screen. Then, select "View My Wallet." On the right side of your Steam screen, you will find the option "Redeem a Steam Gift Card or Wallet Code."
Or, you can directly visit the following link after logging in to your account: https://store.steampowered.com/account/redeemwalletcode