Designing my first game, A Winter's Tale, I wanted to create a simple pixel esthetic. It seems to me that pixel art should be incredibly easy as its simple squares. The truth is however, that I had an extremely difficult time finding or creating square brushes in any software, whether it be simple or complex. I'm sure there are advanced users that would scoff at this, but for the uninitiated this was a challenge. Enter Illustrator's Rectangular Grid Tool. Now you can quickly lay out a grid of rectangles, in varying sizes, and just fill in each one.
Rectangular Grid Tool
The steps I describe are using Adobe Illustrator CC 2014 Release. Start a new project; I always use 1920 x 1080 for the dimension, despite vector scaling, to stay consistent and should I save a standard image, it will be in the HD format. With the new project open, click down and hold over the Line Segment Tool in the tool pallet. Now with the alternate option list showing, change to the Rectangular Grid Tool.
Next to the only real trick to the whole process. With both the fill and stroke colors nulled out, click once in the canvas and drag out your grid. Without letting go of the mouse button, press the arrows keys to add, or remove, rows and columns until they appear square and are the size you are looking for.
Alternatively, you can make this cleaner by instead clicking once and letting go; the tool options dialog will open. Enter identical dimensions for the height and width and then an identical number of horizontal and vertical dividers. Deselect "Fill Grid". This will make your pixels perfectly square.
Live Paint Bucket
Return to the tool pellet and press down on the Shape Builder Tool to reveal its alternatives. Select Live Paint Bucket. Select a fill color and start clicking in your grid to create your drawing. If you decide to erase something you've done, simply draw over it will the fill color nulled. Should you use the eraser, you'd actually erase that part of the grid with it.
A quick way to color in large areas is to create a new layer underneath the current one and color in with the paintbrush. I also recommend adding the colors you use to the swatches panel.
You could also place an image on a lower layer for inspiration.
You can further tweak your piece from here. If you'd like you can select the grid and go to Object > Expand to remove all the whitespace portions of the grid. Also, if you'd like to draw attention to the squares, you can double-click the Rectangular Grid Tool to bring its options back up and check Fill Grid. Then simply select your stroke color.
So this is my easy way of creating pixel art in Illustrator. I hope that I've helped some people or maybe given you something to think about. Have fun!