How to customize a standard encryption algorithm? I want to modify and customize AES encryption algorithm and develop a variant of it. What factors should I consider so that it does not become weaker? What skill sets would be required to undertake this task? How complicated can it get?
The comments on the question are very good. That said, I'll try to address the question(s).


  What factors should I consider so that it does not become weaker?


This is a very important concern that I am glad you have. One of the big strengths that standard ciphers have is that lots of really smart people have looked at them and have not been able to break them. See Schneier's Law -- "Anyone, from the most clueless amateur to the best cryptographer, can create an algorithm that he himself can't break."

So normally the answer to this question would be to publish the details of the modification so that other cryptographers can analyze them and determine whether or not security has been weakened due to your modifications. Based on your comments, however, it sounds like that might violate other design constraints (BTW, I agree with the other commenters that keeping your algorithm secret is a poor choice). So, instead, I'd recommend hiring a group of experts to review the design and point out any fatal flaws. Actually, what I'd really recommend is that you use standard ciphers and just keep the key secret.


  What skill sets would be required to undertake this task?


Years of experience breaking ciphers. There are lots of ways to gain this expertise, but no shortcuts. It will take years of experience. That said, what modifications you make will have a great impact on what experience you need. For example, could the changes possibly create side-channel attacks? If so, you need that experience. The less you change the better and the easier it will be to do the analysis, but at the same time, the less you change, the more I have to wonder why you are changing anything at all. Make sense?


  How complicated can it get?


Depends on what you do. It could get very, very complicated. To the point that you are really not changing things and really creating a new cipher.

For a recent example of an "updated" cipher, I suggest you look at the Spritz cipher. It is an "update" to RC4. Maintains similar structure to the RC4 cipher but has very significant modifications. Also, take a look at this paper which claims statistical weaknesses in Spritz. See, even seasoned pros can potentially get things wrong when updating/modifying a standard cipher. This also is a good example of why publishing your cipher is a good thing.