Create or sign into your Sendana account at app.usesendana.com. Click on the "Card" icon. Select "Activate Your Card" and follow the steps.
Please note that you even though creating a Sendana card is free, you will need to have a balance of at least $10 USD in your Sendana wallet in order to create your card. After you've created your card you are free to spend that $10.
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You can add funds to your Sendana wallet by transferring funds via wire/ACH to your Sendana USD bank account or by having another Sendana user send you these funds from their Sendana wallet.
If you are familiar with blockchains (advanced users only!) you can also top up your Sendana wallet by sending funds directly to your 'wallet address', from any other wallet on the Stellar blockchain.