You can set up an auto-debit with your bank account and set up monthly payments via the same. Henceforth for the first instalment of the SIP, the funds will be deducted from your registered bank account. Once the bank mandate is signed and verified, your monthly payments can be taken care of directly by your bank account. 
Also, keep in mind: As per the announcement by SEBI, the usage of a common fund wallet / account for Stocks and Mutual Funds transactions will be discontinued. For more information refer to the below circular link