Precognitive Artistry:

Lucy has the ability to predict the future using art. This can be done through either drawing or painting. These visions are hard to control, and if she's not concentrating for the visions to happen, they're spontaneous. 

When these visions happen, she enters a trance of sorts and she draws a scene of the future on whatever she can, whether it be drawing or painting, it doesn't matter. 

These visions aren't always set in stone, since the future changes depending on what a person chooses. More often than not, however, if the vision is strong enough then it's more likely that it'll happen. 

The visions on paintings are very vague however, and she doesn't always know what's going to happen and can't always figure them out. However, with the X-Men's help, she can make a solid guess on what's going to happen. 

Before a vision starts, she can see it on the canvas or whatever she can draw/paint on vaguely and the trance starts. Once it does, she can't be stopped until it's finished, and she can't remember doing the action. Sometimes these visions happen in a series of events that she can figure out more easily, but other times it's simply just one big piece that she needs to spend a certain amount of time deciphering. 

She doesn't always need a canvas to predict the future, other times it could be in class and she psychs out and she sees a vision in her head. The drawing/painting part mostly puts a visual for everyone else, and sometimes herself since she can't always remember the visions.

The deciphering of the visions are limited to her artistic abilities, so it's good thing that she'd been painting and drawing her whole life cause it would be very difficult to figure out what each vision means.