F is the subjective Judging function. anytime you make up your mind (you think it's T) about a subjective value of something you use F (not T), and if you're an INTP you use Fe, because your Ji is filled with T-data
if that doesnt help, alternatively:
if you just stop using Ti, Ne, and Si, you automatically fallback to Fe. with a little practice this will stop looking like
how do you stop? Ti and Ne are per definition things you care most about. just say 'i dont care' let it go. it's like the mental part of meditation without all the leg-in-knot-bending. basically if you made a judgement about something and you dont care about the judgement (find it boring) it's a good bet that was a Fe judgement.
remember F is a
rational function, just like T. it's just subjective is all. Fe = logical deductions from subjective values of some_guy.