Modded clients

Hi all

I was thinking about making a moddified cliënt for another game but I have no idea how to start with it. ;s
I ask it here because some people have made a modded AL client so maybe they can help me out.

So for the pro coders out there:
- How do you begin making a modded cliënt?
- Is it the same like the minecraft modded clients or not?

Thanks!