Research, research and more research
Look up custom builds similar to what you want and see what parts others tend to use. Look up reviews for those parts and see if you like what you find out. Make sure you read up on different standards for parts and check if the new parts will match what you already have on your bike. And don't hesitate to ask if you aren't 100% sure about something.

It does take more time that way, but I assure you - hand-picking parts for your own bike is incredibly rewarding and it will truly feel like your very own bike once you are done. Good luck and do share with us once the build is ready!