Log in to your Phantom wallet
Click on Receive
Select Polygon
Copy the Polygon address from your Phantom wallet
Go to Step 2
Log into your Sinbad wallet
Ensure you have a balance. (Your balance is already in USDC)
Click on Cash out
Select USD Coin (USDC) option
Enter the amount you want to withdraw then click on continue
Paste the Polygon address you copied in Step 1 above