Transfer of business

You have to apply for new registration of the proprietorship business. 

The new gst registration will be on the PAN of the proprietor.

Need to Obtain GST registration for Proprietor firm .

During the Registration process need to provide Reason (merge) & GSTN of Partnership.
And while surrendering the GSTN of Partnership Firm need to Give reason for cancellation of Registration by selecting the "Change in Constitution" thereafter provide the GSTN of Proprietor firm.

Do Prepare a transfer business Deed .

After getting new registration i need to file any return? as firm is under composition scheme

Yes till before surrender you are liable to file return