Does this Southwest Airlines gift card have fees or an expiration date?

No. The gift card has no fees and does not expire.

How can I redeem this Southwest Airlines gift card when booking online?

After selecting your flights and entering passenger information, choose “Apply Travel Funds” in the payment section. Enter the gift card number and PIN, then select “Apply Funds.” If the gift card does not cover the total, pay the remaining balance with another payment method.