Just in general, take things slow. If you rush, the quality will suffer, you'll make mistakes, get frustrated, and quit.
Measure things you need multiple times as well, just to get an accurate number before you start cutting things apart. Check and double check your lines and patterns before you cut as well.
i'd suggest actually getting some super cheap plain old cotton or whatever to test your patterns on before you get straight into the faux fur. That way you can see how things go together, and correct any issues that might come up.
When building anything using foam (assuming you're making a foam based head), use references! Have someone else on hand to redline pictures you take of your base! Things always look so much better if you have a reference to look at, and more sets of eyes are more likely to pick out any symmetry issues.