I can only speak to Fe, as it is my inferior function. I find that getting out amongst people, where I can exercise it, works best for me in developing it. A small amount of alcohol can work wonders there too, since I am an introvert. I'm far from an expert in employing it ... after all, it is my inferior ... but I enjoy trying.
The process of using Ni is easiest to conceptualize as creating mental models that you can manipulate as if you're standing from the outside looking inwards.
Outside perspective so you can rotate the model and view it from multiple perspectives until you can find the overreaching principles behind the whole.
The way I see it, the end goal in my Ni is the unity of all knowledge into a whole. So how do things interconnect, what effects does changing one aspect have on the others, cause and effect is always on my mind. The relations also focus more on the intrinsic properties of the object rather than the physical.
Lastly we have play. Ni is like Ne in that all exploration is playful. How does everything relates to this one thing? It doesn't matter how obscure the connection, as long as there's one Ni will be happy.
I am very in-tune with my Ni. Occasionally I encounter a situation I perceive to be irregularly special. At these moments I hear my Ni and know what this person wants me to say to make them feel good. Usually this results in me feeling awkward and nearly afraid of saying what needs to be said, as if fulfilling what this person wants me to say would be a submission. It feels like I'm exposing myself- quite literally like feeling naked in the room.
To others this shows as lack of emotion, feelings distant. Almost antagonistic, I'm ostensibly self-centered. It's as if I am so enveloped with a need to know and be myself that I fear the whole unity-thing that is love and other people. I'm starting to get the dark humor on this board that surrounds the ENTJ...
For the ENTJ that requires consciously setting aside time for solitude and reflection. Keep a journal, take a meditation class, take walks in nature (usually more appealing to ENTJs) and listen to your thoughts.
During alone time, practice using metaphors or analogies to reconsider a problem you're trying to solve
Join a book group that concentrates on fiction to look ath the themes, motivations and emotions of others (this will actually help Fe as well)
Take time for imagining yourself 5 years from now. Will your current plans get you where you really want to be?
Take up a solo, creative hobby that has no competitive aspect. Poetry, gardening, fiction writing, artwork are all possibilities
Fe is harder, obviously...I'd actually suggest that you make a study of what aspects of Fe would make you a better person. On which do you want to concentrate? Gaining group consensus so your plans (maybe with adjustments after gathering input) are carried out more efficiently? Understanding teh motivations of others? Recognizing others needs? Then chart out how you know when others have accomplished these things. What do they do? What skills do they use?
Analyze what wins people to others' point of view. The goal isn't to manipulate but to understand how to make it appealing for them to join up with you
Practice telling others what you value about them
In conversation practise holding your tongue until you think of something positive to say before any critique
Acknowledge the efforts of others as progress is being made, not just at success
Reflect on realistic expectations for others.
Actively seek the input of others, even those you don't consider experts. Maybe even the person most likely to challenge your views.