It's stupidly easy. If you place the paper bag in the bagging area, there's a prompt that asks you if you placed a bag in the bagging area. If you place the bag then very quickly scan the first item within a second or too, it still looks like you scanned it (makes the beeping noise) but the machine is so preoccupied with asking if you put a bag down so it can zero out the scale that it just completely ignores that first item, lmao.
If the self checkout person notices (they won't because they're too preoccupied with checking IDs for alcohol purchases) the you can just play it off like you were in a rush and didn't notice