(Please note I have seen this from observation and am not actually a checker yet, but I'll do my best.) It's honestly just like doing bagger all over again, but behind a register and with less of a load. IMO i think all new cashiers should start on express first, then move to a larger check when they are sufficiently trained n ready. But maybe just wishful thinking. Ok here we go.

Bag as you go in small groups if you can help it if you have your 15 items or less sign up (which you mostly will be, unless you get that jackass who comes through anyway). (Meats, frozen, etc.) This will help keep your ELMS score steady and high.

If you are underage (<18-21, depending on state) and have alcohol, either call out for assistance immediately or as soon as you get the rest of the order done as your ELMS is still ticking. (normally if I'm working around a younger cashier who i have scanned for before, if they aren't able to get a CSR first, I'm usually the one they will call, if not another checker who is free if i am busy, and I redirect it to someone else.) While you are waiting, have the customer present their ID to you and have it ready to hand off to the other clerk to verify and ring the stuff through.

Ask the customer if they could have any coupons when completing or nearing the end of the order, or have your coupon book ready to go on the side desk.

Memorize your PLUs or keep a cheat sheet handy, or there should always be a book listing them at the counter or in your lower drawer. For some produce / water, all our registers, even self check, have common and some less common produce / candy / organic items listed below the screen to make it easy for the cashier / custy before you have to go hunt for them. Also if you sell ice, keep your spare tags handy on the desk or on the small little counter wall to stick to so you can swipe one quickly when a customer wants a bag, they can go get it while you finish, or can do so after the order.

Don't be afraid to ask for help, it's how we all learn and grow. If you need extra money or anything for your till, dial Service Desk from your desk phone and let them know what you need. (Do not call out loudly that you need some. If the CSR is nearby you, call out to them and wave them over to you and tell them quietly. At least at my store, this is an LP / Management issue that can get you in trouble as you are basically saying "hey i need money, look at me." Same applies for a till / change dispenser drop / change, do it discreetly, as if you call those out loudly, you will get in trouble with LP / Management much quicker as that puts a thief on alert and makes you a potential target for robbery. At my store, our phones are labeled with both Mgr on Duty portable, Mngr office, and Service desk listed as direct buttons (speed dial)

This is about all I can think of right now. If theres more, i'll update the post. Hope this helps.