There are a few ways that people do that. Sometimes they cross their chosen breed with another breed that has that color pattern. Other times they start with the correct breed and cross colors/patterns to bring in the genes that they need. Either way, it is quite a lot of work both in planning and execution. I am starting a line of blue laced bantam Cochins this year. Playing around with the chicken color genetics calculator can help, as can reading through other threads. Many times there are others already working on your chosen project. How much do you know about genetics in general? That is a good start.