First to make highlights for eyes or say the pupil, I would roll the small amount of fibres in my fingers just to keep the wool together. Trying to wrangle individual fibres is a nightmare! So roll the fibres into a tiny ball in your fingers then needle it in place. It will take practice, but rule of thumb is to use more than you want, as everything you needle usually reduces in size.
With linear details, I usually apply the same technique. For example a stripe on a tabby cat: I would pull out some fibres, then roll them together to make like a teeny tiny rope (this is hard to explain in words!) along the length, so that the fibres stay together and you're not dealing with individual fibres. Then needle it in place.
Best thing to do is to needle felt a ball and practice on it with eyes and lines etc. You'll get the hang of it, just practice, practice, practice.