Welcome to this forum, and to mystery shopping.
You asked for some tips: Hum I would say don't wash whites with colors. Or don't take wooden nickels.
I would suggest you ask a question, and we will be more than happy to answer you.
OK, to be serious for a second here. I would say that the best tip I can give you is to get yourself a decent smartphone. I use an iPhone 6S+, and it helps me a lot with my MSing work.
More on that later. Now it's your turn to ask.
Please feel free to ask any questions you have regarding doing shops. A smartphone with the app makes the shop and reporting so much easier. Make sure you download the app. Also, review the instructions/directions very carefully and completely before you accept a shop assignment. Read the instructions again and again before doing the shop, so you don't miss anything.