Adding joints wasn’t much of a difficulty for me, but I ran into crises with the weight of the skin (pictures below). This probably took me the most time to solve as I was facing the same issues over and over again. At first, I was too rough with painting skin weights and had to smooth it more. This was part of the problem, but it didn’t fix everything. I spent some time blending them and it made the poses look more natural.
My main trouble though was the jaw rotating with the head (should be just the head). It was my mistake that I keyed the wrong parts of the head in the timeline. This was most likely because I wasn’t paying enough attention to the timeline and keyframes. Next time, I’ll be sure to make sure all my keyframes are set up correctly, and if I run into another similar issue, I’ll be sure to check the timeline first.
While working on blend shapes, I noticed that skin weights change and mess up the model. I solved this problem by locking skin weights. I feel like I miss a step sometimes and it causes me a lot of trouble later on, but at least now I understand it better.