Image Source : http://www.flickr.com/photos/andryshuzain/118971151/
If you are worried about not being able to draw images quickly enough,one way to start is to pencil them lightly beforehand and then draw with a marker in real time. This will give you confidence in working in real time and also provide the support you need at the beginning. It also gets you drawing the images. Sooner or later, you will leave the pencil drawings just as you left training wheels while learning to ride a bike.
Milly R.Sonneman in Beyond Words, a Guide to Drawing Out Ideas