I think the majority of us make customs because we enjoy the process of creating a new pony, not necessarily just to sell them. The few customisers who have regular sales and commissions have years of experience customising.
As for advice, I agree with thetailrustedtealeaf, start by checking out all the tutorials that are available online. I also start my customs with a specific idea and colour theme in mind and I know my strengths and weaknesses - I'm better at sculpting, therefore I tend to focus on sculpting extras on my ponies rather than painting. Good luck!!