These days, adding drives are simple to do, following the advise given above.
Old Phil makes a point often overlooked by too many, that ensuring that all power is removed & drained from the computer, as well as grounding. This includes the removal of the Ethernet cable, if installed. After disconnecting all power, make sure to press & hold the power button for at least 30 seconds, this flushes any leftover lingering power.
Never, under any circumstances, attempt to perform such repairs/upgrades on a carpeted surface. This increases the chance of damaging the motherboard & other system components, such as the GPU, CPU & RAM.
Other than that, adding a 2nd drive can easily be a 5-10 minute task, the most time used in preparation. The physical act of adding the extra drive can only take a minute. It is good to have a 2nd drive installed, for more than one reason. It can be split into partitions, one can be the backup partition for the "C" drive, eliminating the need for an external. All of my computers with SSD's has a separate drive for that purpose & Data.