It is very easy to do and I have done this with my MacBook Aluminum using an SSD I bought on eBay. Works like a dream. It will take about 5 minutes to do, as someone has posted.
You only need a single thin philips screwdriver as well as a torx one in order to get the brackets off the original and put them on the new HD. If you are lucky you can probably get both as part of a repair kit at the dollar store (not the best quality, but will do if you only use it a few times in your life).
The risk is absolutely minimal as you can expose the HD by removing the battery door - it will be just right there. It is also isolated from all other components so there is virtually no chance to damage anything while doing this. You don't have to be a computer genius or whatever to do this.