When creating shapes that require absolute precision the Align panel (Shift + F7) is your best friend: this is the best way to get absolute symmetry within your work. Just remember to specify whether you’re aligning groups of shapes to one another or to the artboard in general.
Group any elements that appear in pairs – such as eyes – and use the Reflect tool (Object > Transform > Reflect). Make sure you reflect vertically, and set the desired distance required between the eyes using the arrow keys. Group similar objects to make navigating the Layers palette easier.
Once all the key elements are in place it’s time to think about colour. Take your time when picking the colour palette: I tend to work with a limited palette as I feel it gives more balance to the overall illustration – but working with a small palette means it’s even more important that you choose the right colours.
Now apply your chosen colours to the illustration. Make sure that the right colours are in the right place: this is very important – placing colours badly can break the flow of the illustration. Use lighter colours to highlight, darker colours for the base shapes and midtone colours to add depth and feel.