Step 1) Begin with an introduction. A good introduction should talk about how you came to go to the mechanic in question. Talk about what the problem was with your car and why you selected this particular repair shop. If you had heard anything about the shop beforehand, this is the place to discuss that.
Step 2) Tell the reader what was good and bad. The worst experiences with car mechanics may still have their good points, and the best experiences may still have their drawbacks. Write in a balanced manner that is totally honest about the pros and cons of the car mechanic who you went to. It not only tells the whole story, it also increases your credibility as a writer.
Step 3) Refer to specifics. When writing your review, it is important to talk about specific things that happened or didn't happen when you were there. Writing in vague, general terms doesn't tell the reader what they need to know to make an informed decision.
Step 4) Keep the focus on the business. Your review is of the car repair shop, not the personalities involved. Make sure to focus on the business experience and avoid the personal wherever possible. It is, however, worth noting that customer service is part of your experience as a client and should be included.
Step 5) Compare the mechanic to other mechanics you have visited. Good, bad, or indifferent, the car mechanic you are reviewing exists in a broader world of other mechanics. Talk in general terms about how this mechanic measures up to other mechanics you have patronized in the past.
Step 6) Draw conclusions. In the final analysis, you either would go to a mechanic again or you wouldn't. Make conclusions about the mechanic that help the reader decide if they should do business with the mechanic or not. Use your concluding paragraph to weigh the good against the bad.