Take a deep breath! You’ll do great! Your SM wouldn’t have chosen you as a trainer if you didn’t have potential to facilitate learning and foster growth.
You should have a big packet that walks you through all the actual things to go over each day, so you won’t have to wing it.

Things that will help:
While they’re going over their computer modules, if you’re able to, take a minute to collect yourself and go over what you will be training today. Brush up on anything you may not be following up to standard (beverage routine, cs cycle, whatever)
Once they’re done with their modules, ask them to give you a quick little recap of what they learned! I find it’s best if you can have them do most of the talking at this point.

Your first day should be customer support cycle, which will be a great time to show them around the store!
Ask them early on in the shift what helps them learn best. I’ve had trainees who need to watch me do something twice before they feel ready to try themself, and trainees who just need me to explain it to them verbally before they’re ready. It just depends on the person.

Don’t forget to introduce them to everyone in the store. Make them feel welcome! You may have to introduce them more than once lol.
On that note- you should be scheduled to take your break together. Take this time to get to know them! It’s scary coming into a new job, and you’re going to be the one person they will know and the person they can trust the most here. It’ll be good to know them on a personal level.

Repeat, repeat, repeat. Once you’ve gone over something, go over it again. Then have them explain it back to you. Starbucks shoves so much learning into so little time. You’ve gotta make sure you’re reiterating this information otherwise it’ll get completely lost in the sea of facts and sizes and pumps that we are expected to memorize in just 3 weeks.
Make sure they know that it’s OK to mess up. It’s ok to spill things. It’s ok to ask questions- to you and to other baristas. Nobody is perfect and learning takes time!

Praise them for what they do right as much as you coach them for what they can improve on.
And don’t feel like you need to go fast. even with a line around the building. The store will survive if it’s busy for a few hours and you and your trainee are working at a learning pace. It’s better that you build a solid foundation of knowledge than sacrifice that learning opportunity for a few moments of profit/peace. Your obligation is to your trainee and their comfort, not your stores profit margins.

You’re going to do great!! I Believe in you!
If you really want to impress your SM, take more detailed notes in your training packet.