When reading the fluff, whatever particular psychic discipline a character is said to have is usually described as being the most difficult. From a more practical perspective, which disciplines would seem to be the most difficult, and from a narrative standpoint, what are some things that might be worth describing to my players as reasons for failure, some bonuses or penalties assigned for distractions and environmental factors. More specifically, what do you think it might be like to actually manifest a power from your mind, and what would be the best way to describe it in narration. For example, I think telepathy would be incredibly difficult to control; sometimes its hard enough keeping all my thoughts straight, let alone adding a foreign mind to the mix. But describing how controlling fire, inanimate objects, meat and bone, and essentially fortune telling are all things characters can do. How do you describe to your players what goes on during the manifestation process?

As far as modifiers go, you should probably just leave the difficulties as they're stated in the basic power. As far as I know, manifesting psychic powers shouldn't really be subject to many special modifiers.

For describing them, I'd recommend going for a horror-based description. Play up the fact that these powers are unnatural things pulled from the warp, and that the willpower needed by psychics to use them is less about force of will and more about withstanding the forces they're pulling out of the aether. For Divination, describe some horrific visions seen by the psyker, such as the rotting corpse of those he loves, the death of the emperor, and other images that flash through his mind. When using divination powers with physical world effects, describe how the psyker moves almost by accident, his body moving with terrible grace and clumsiness. Telepathy should be similar, with the images seen by the psyker being from the targets mind, or the targets feeling a scratching inside their skull or similar. Play up the whole mental violation angle, but don't associate it with sexual assault or anything, as that is not appropriate for the game. Pyrokinesis describe in terms of temperature changes, te psyker feeling as though he himself is burning, the smell of smoke and burning flesh. Telekinesis you can describe the psyker wrapping his will on the world around him, but suddenly being all too sensitive to that world. The sharpness of metal, the iron taste of rust, the booms of footsteps, all of these things within his reach, but all of them so painfully LOUD to the senses. Basically, for telekinesis as the he reaches his will out to his surroundings, have the surroundings push back on his mind and senses like a flood. For biomancy, just do yor basic body horror of stretching breaking bones, tearing flesh, horrible visages, things like that. Have the psyker be all too aware of his own body, down to feeling the very blood cells moving through his veins.

Keep in mind that when describing these they should be meant as flavor, with the implication that the psyker always feels these things when manifesting a power, and that a failed roll just represents the psyker being overwhelmed by them.

Right. Maybe I didn't explain it well. I don't intend to inflict penalties as a result of narrative things, but the other way around. I'm looking for narrative ways to describe some of the things that go on while manifesting powers. Those are all very vivid descriptions, I like them and will incorporate them.

If you want some ideas for Biomancy, I reccomend checking out a few of the Animorphs books by K.A. Applegate. Some lovely body-horror moments by the protagonists, and their transformation sequences are anything but charming. Certainly enough to give ideas.