No it is not difficult, you just need to obtain the correct memory modules & then take some care when inserting them.
Apple have guides for older Mac Pros, if you take off the door you will see instructions are also printed inside that indicate what order modules need to be added (they normally need to be in pairs).
Mac Pro (Mid 2012 and earlier): How to remove or install memory - Apple Support
You need to take care with static electricity when handling the modules, but that is not difficult unless you run around like a mad thing when doing the job. Grounding yourself on the case is normally enough before you remove the power connector as shown in Apple's guides. You also need to avoid using too much force but hopefully that is apparent to you already 🙂.
Many online RAM providers will sell modules for your model, just take care to use reputable suppliers. Many people here recommend OWC, http://OtherWorldComputing.com/ I tend to use Kingston modules which are easier for me to find locally.
If you are still unsure, seek out an Apple store but Apple did try to make it a painless process in these models.
P.S, you don't have a G5 - that is the very old model with a different processor (that can't run 10.11). Look at the 'Apple menu > About this Mac' to see the model name.
