Perhaps reach out to summer students from previous years to gleam more info about the process, as I imagine it varies drastically depending on the small firm in question, the practice area, etc.
My experience as a summer student at a big full-service law firm:
Luck: 10%
Skill: 20%
Concentrated power of will: 15%
Pleasure: 5%
Pain: 50%
Partner won't remember your name: 100%