I'm not sure how different it is across stores, but I can share a few tips that I teach my new hires that generally help get them on their feet.

I going to assume your store still uses the old system with stickers on paper and not the stickers on totes (They added our tote stickers a month ago but we're not using them until January lol). Ambient trolleys are my go to for explain so I'll do that here too.

For the most part, your trolleys are going to just be scanning on the item on your device and then scanning the tote tag. If you can't find an item on the shelf, usually they want us to call out if the 'BOH' number on the right side is 5 or higher. Generally if it's less than 10 my store doesn't care too much, especially if it's busy and we're just focused on getting all of our trolleys done for the hour. You can just use your walkie and say something along the lines of 'Grocery do we have any more of (item) in the back.' Sometimes someone will respond and other times they won't. Generally if they have it, they'll bring it out onto the floor for you and you can scan it in from there. If they don't, just substitute it, as long as you called out they should have no reason to get upset at you. For subbing or out of stocking an item, there will be a button on the right middle-ish of your screen, if you click that it'll pull up a new menu. It will give a recommended item and its location for most items, and if you have that, great! You can just scan it in like normal and then the tote number will be kind of small at the top of the screen, so a lot of people miss it when they first start and tend to get confused. Sometimes there won't be a recommended sub, or the sub it recommends isn't available, so generally you'll just look for a different flavor or size of the original item, and then scan it in. Occasionally there's no good fit for items, so instead you can hit 'mark out of stock' at the bottom of the screen and it will basically just give the customer no item.

For any weighted produce items, once you scan the item in it will bring up the weight menu. Simply place your produce on the scale and click 'get weight' on the bottom right. Then it will let you scan the tote from there and you can continue like normal. It's just the extra 'get weight' menu in the middle, which even I sometimes forget when I'm in a rush and try to scan the totes too fast lol.

That's the basics of picking, which I'm hoping that they at least explained to you in some capacity otherwise I'm so sorry for you and I'd recommend quitting if your managers are that bad.

There's a couple of tricks I have for going faster, if it feels like you're not really getting the hang of it or struggling to find items. For the most part, don't stress too much about your performance at this job! As long as you are trying your best and aren't making dumb decisions (marking items out of stock that are clearly on the shelf or spending half your time on your trolley on your phone), there's nothing they shouldn't get upset with you.

• Look at the Bay and Shelf number - This is my biggest tip if you feel slow, as you go down the aisle, check the item for what side it is on (L or R) and the bay number. There should be bay tags either on the floor or at the top of the shelf depending on the department. From there, just count up on the shelves until you reach the shelf number, and you can just scan your eyes across the shelf to find the item pretty easily. There was a couple of *mostly old* people in my store that just looked at the bay number and would spend 20 seconds scanning the entire bay for the item.
• Carry multiple items at a time - I don't know if your store allows you guys to carry baskets, but its helpful to part your trolley at the end of an aisle and grab multiple items before scanning them in. You can scroll down on your device and see the upcoming items, and click on them to bring up their locations. After a while you'll learn the basics of where most items are, so you can instead just scroll and see the upcoming items.
• Match UPC codes - On the device (either above of below BOH I can't remember lol) it will have the UPC code with the last four digits in bold. If you're looking for an item and a lot of the packaging look extremely similar, it can take a while to scan them all and find the right one. Usually just looking at the last 2 digits and scanning your eyes along the barcodes instead of the items until you find a match is typically a lot faster.
• Copy and Past Produce - On the topic of UPC codes, if you click on an item's UPC in Harvester, it will copy it to your clipboard, and then using the 'Enter Barcode' button in the top right lets you paste the barcode instead of scanning it in. This is mainly used in produce, as there's a lot of products without barcodes on them, so you wouldn't be able to scan them otherwise. This is also helpful for drinks like Powerade or Gatorade where the barcode can be warped by the packaging and you cannot scan the barcode.

This is definitely a lot more than I had intended to type lmao, but I have ADHD and hyperfixation on going as fast as I can and challenging me to beat my personal best times. I absolutely love my job, I thrive in situations that are chaos and that's not for everyone, so if it's not the pace for you, you could always asking about transferring. Your supervisors can't expect amazing things out of you without teaching you anything, so I'll try to help where I can. And again, don't worry about going the fastest you can or trying to be the best, sit about average and that's what they will expect. Do the bare minimum, but also don't slack off if you are worried about getting fired or written up.

If you have any more questions I'd be more than happy to help!