“Can you develop all eight functions? If so, how would one go about doing that?”
That would be an excellent idea if you want to go nowhere in life.
I would not recommend trying to develop all eight functions, and here’s why.
You have four functions that you utilize on a regular basis, but occasionally, when you’re extremely stressed out, you can lapse into using all opposite functions (aka, your shadow functions).
For instance, I as an INTJ, typically use NiTeFiSe, but when I lapse into my shadow functions, I start using the functions of an ENTP (NeTiFeSi). Shadow functions pop in on rare occasions to give you a moment of surprising, out of character insight, but typically, they pop in to make your life miserable.
You’re naturally keen to using your four type functions, and over time, you get better and better at using them. Everyone is born with natural affinities. If you deliberately try to use your shadow functions, you’re not going to end up superhuman or good at everything. In fact, it’s going to take you in the opposite direction.
You’re going to end up doing a lot of things but not being good at any of them.
On the other hand, if you focus mainly on developing your four natural functions, you’re going to be much more successful in life. Geniuses are not necessarily balanced in all areas. Albert Einstein was a genius academically, but he couldn’t tie his shoe laces.
Watch this video if that didn’t make sense (or if you’re an auditory learner).