Dell has not published the procedures for installing an additional SATA hard drive, the Inspiron 3847 has a second 3.5" bay, for the second hard drive next to the primary hard drive, there should be a set of rails located in the bay, two 6-32X1/4, phillips head screws are needed, to attach the rails.
The procedures on pages 26 to 28 of the owners manual should give you an idea of what is required for the installation.
You will need a SATA data cable there should be a spare SATA power connector inside the case.
After installing the drive enter the system setup and check that the hard drive is 'enabled' [on] and recognized.
For Windows to recognize the second hard drive, it must be partitioned and formatted.
Go to Control Panel, Administrative Tools, Computer Management, Storage, Disk Management, right click new drive, select format, then you can assign a drive letter.