First you need to deactivate the axes so press "2nd" the "Zoom" and move to AxesOff and press "enter".
Then you can play with pen by pressing "2nd" then "PRGM" to enter "DRAW" menu.
Select last item ("Pen") then press enter. Now you can move your cursor and when you press enter, the pixel becomes black. You can store the result using StorePic (look at the ti 84 plus guidebook)
Have fun !