Gosh its such a long discussion about this. But you can't really improve your Inferior Function. That's why its called Inferior. It sort of operates outside of your general awareness and locus of control. The best we can do is learn to better integrate it into our lives and that takes time (perhaps a long time) because the Inferior function represents everything we have consciously chosen not to be. So to get someone to really deal with it, is sort of like asking them to become something they have spent their entire life avoiding. You see the difficulty here.
Functions are a process, not just a tool like "right now I'm using Se and tomorrow i'll be using Fi," etc. They're much more like mindsets or perspectives that help your ego point in its desired direction. The inferior function basically is the monkey in the works. The troublemaker that threatens to mess it all up, and as such, even Jungian psychologists are warned never to directly engage one's inferior function directly. It's often their sorest spot, like putting alcohol on a wound, and quite commonly the function through which any latent effects of trauma or psychological pain will come through. So its not as easy as saying "lets pay some bills and go to law school to learn how to be a Te-type." Getting from dominant Fi to Te is more like unwrapping a mummy. Must be done very carefully and with great prejudice by the person and the psychologist.
Anyways theres a lot of information on this function I can point you to, but suffice to say learning to deal with the inferior function is probably one of the single great challenges of life and often people are well into middle age before they've really given it the time of day. Most people simply try to ignore it, pretend it doesn't exist, or project it as something that other people do and not them.
I should also say its not that Fi-doms don't use or have Te. Quite the contrary. It's just that Thinking is very raw and often easily jumps to conclusions or exists in a world of black and white (its not the highly developed nuance thought of Thinking types). It's much more the level of Thinking of say an animal (not my quote but Von Franz) where if you tell the dog don't poop on the floor, the dog doesn't know whether or not you mean, the house floor or all floors. Now certainly people are more intelligent than that, but we've all come across people who don't seem to reason, in terms of human issues, with much more thought capacity than that. The ability to conceptualize and then think something through. More often Inferior Thinking types just throw up their hands and cry from a Feeling perspective about how hard it is for them, or that the rules are too impersonal, or that they don't get it, rather than engage their own inferior sense of Thinking.
To add to @LiquidLight 's thoughtful advice, being in your inferior function has to be short-lived or it will drain you. When you say you want to learn to use extroverted thinking, that tells me you just want to learn to be more organized and methodical. In my opinion, unless you have a strong will to learn time management, executive functioning, and organization, those skills will not help you in terms of self-improvement. What you have to do is embrace your value in Fi. You think it is useless? You ought to work on your own self-expression instead. INFPs live and breathe it! You need to overcome your fear of making mistakes so that failure opens up new areas of opportunity which is what should excite you in the first place rather than ruin your identity. Only after you have done enough self-expression will you want to take yourself to the next level and support a higher cause, proving to others that you can be practical and deliver on what you promise.
On another note, it may be useful to add why you want to work on areas that are attributed to Te. I don't know what circumstances you are in or what obligations you have...
Yeah, I think @LiquidLight's point brings up the interesting issue of what thinking and feeling truly entail (rather than all of the shallow stereotypes about Ts being "intellectual" or Fs being "emotional"). Fundamentally, thinking comes down to having coherently structured thoughts (so, that's why the function will manifest as logical in nature, although it really says nothing about the interests of a person and shouldn't), while feeling comes down to having coherently structured feelings, or in other words, evaluative judgement (e.g. good/bad, what makes someone feel one way vs. what makes the person feel another way, consistent reasoning off of personal values so they know how things will effect them and others, etc.). For high thinking types, judgements ultimately come down to what the person thinks about something, while for high feeling types, judgements ultimately come down to what the person feels about something. If a high T type is motivated to act on a feeling, they will usually rationalize it via logic, while if a high F type is motivated to act on logic, they will usually rationalize it via feeling.