I'll write it step by step from an HR perspective, the first hire creates the same panic in everyone.
Define the job first, not the person. Note down every task you do for a week. Then mark the ones you want to hand over. Those tasks are your job description.
Then clarify the working arrangement: full-time, part-time, which days, which hours. In the boutique business, weekends and working hours are the most contentious topics, so write them down from the start.
Discuss the financial and legal side with your accountant before hiring. There are obligations like workplace notification, insured employment registration, payroll setup, and having a written employment contract. These are technical issues and every situation can have its own specifics — don't proceed based on hearsay.
To find the right person: don't ask abstract questions in the interview. Instead of "How would you describe yourself," give a real scenario. "A customer wants a return but the tag is removed, what do you do?" The answer will tell you about their working style.
Finally, use the probation period and actually evaluate during that time. Most employers see the probation period as "a period that passes automatically" and then work with the wrong person for a year.