The various source selection methodologies are covered in great detail in FAR subpart 15.1. It is too much to explain here so I recommend reviewing subpart 15.1. The applicability of FAR Part 13 procedures is best understood by reviewing FAR 13.003. You will find that selecting sources for simplified acquisitions is "simplified" when compared to FAR part 15 acquisitions. Again, reading the FAR is necessary to understand the procedures and implications.