A simple online application will serve the purpose. Their application has a column where details of existing credit card with credit limit are asked. I applied online and received the card (along with one add on) within 10-12 days from the date of application, with a good Credit Limit. Before you start the online application procedure, make sure you have soft copies of following documents:-
1. Photo (In my case they got it from the eKYC & for add on, I had to upload).
2. Address Proof (Electricity/Tele bill if address on the Aadhar is different).
3. PAN
4. Aadhar
5. Salary Slip/ITR