1. Keep doing what you're doing in terms of studying the guidelines carefully and preparing for your shops.
2. Whenever feasible, try to do a shop earlier rather than later (within your shop window). For certain shops, you may have the luxury of re-shopping if necessary.
3. Communicate with your schedulers and MSCs if possible and when necessary. Don't be shy about asking for help or clarification - hopefully BEFORE a shop.
4. While companies and schedulers would love perfection from shoppers, smart ones know we are all human and WILL make mistakes. Be honest and own up to any mistake. Try to take feedback as constructive criticism. Learn and improve.
5. Engage in a mental exercise before a shop if you're stressed about actually performing the shop. Imagine doing the shop. Go through each step.
6. Just do it. The more successful shops you have under your belt, the more confident you will be (and, hopefully, less stressed, too).
Good luck.