You can subscribe to Tuned syndications through the invest page or if you have the syndication link. Once you are on the syndication page, you can see the below image where you have a button to subscribe to the syndication.
As soon as you click on subscribe button, it takes you to the payment details screen just like below
Click on Continue to Payment Details, and it takes you to the next screen where you can add a credit/debit card if you don't have one added.
Click on this screen, and it takes you to add a credit/debit card. Just fill in all the details, click on confirm and your card is added to the tuned account dashboard.
If you already have a card added it gives you a different screen like below.
You can also replace your card from this screen if you click on Replace Card. If you want to proceed with the transaction just click on Continue Payment. It then takes you to the below screen.
When you click on okay, it presents you with another window, where you can select your exchange API keys, set the allocation, and create a live run.
Click on Start Live Run after selecting your exchange API keys, set the allocation and you're good to go. You can view the live run in the Live Tab.