How to upgrade SSD from HDD for my mac book pro
You should look at the ifixit HDD replacement guide for this model. This will show you exactly the steps you need to take, including 'unplugging' the logic board from the battery.

You may want to buy a USB to SATA connector cable. This will let you connect the new SSD to the Mac, so that you can copy the contents from the hard drive to the SSD. You need to use cloning apps: either SuperDuper! or Carbon Copy Cloner to do this. 

If you have a Time Machine backup, you may be able to restore this to the new drive by booting to Recovery (command R) and then restoring the backup.

Then swap the drives, and you can still use the HDD as a temporary external drive with the SATA cable.