Everything that's been said, plus... I'm throwing out a few things I have seen in guidelines for restaurant shops.
Be the opposite of what you see on Yelp. Mystery shopping clients want specific information that they can use to confirm their staff are following the training they are given, not opinions.
Evaluate the food and drinks for what they are, not whether they suit your tastes. i.e. If you ordered something advertised as "spicy," but you don't really like spicy food, don't complain that it was spicy and you didn't care for it. However, if the menu stated it was "mild" and it arrives spicy, that's something to mention.
Keep emotions and personal things out of it. (some surveys do ask for emotions, but very few)
Don't compare it to other restaurants. Don't compare it to other visits at this restaurant. Talk only about what you observed during this visit.
Timings are really important in restaurant shops. Make note of how long it took to get your items after ordering them. Since this is a sample, you probably didn't time it, but it would be good to note if you can remember or if there was anything notable (things arrived quickly or slowly).