Not sure how to answer this. Hopefully she sent leaves that will produce new plants (some begonias are not able to do this).

Start with clean pots, soil, scissors, knife, etc. I recently started washing leaves with soapy water to remove a lot of the germs present - I think this may help.

That being said there are many posts on this site alone that show how to propagate a begonia by leaf cutting. Just use a good potting mix or straight perlite. If the leaf is small then cut the stem leaving about an inch attached. Push this stem into the moistened media to the leaf base. Cover with a lid or a baggie - make sure it never dries out though. It should root in a week or two and then start producing new babies. If the leaf is very large then the best bet is to trim the outer edge of the leaf, cut off the stem, and slice between the major veins (like you were cutting a pie into pieces). Push the base ends of each wedge into the moistened media and treat like the smaller leaves. The big thick leaves may need treating like a succulent because they may rot faster than the smaller thinner leaves.

Here are some cuttings I did back in April/May using Jiffy peat pellets. Works pretty good.

Thanks! I got them 3 weeks ago, I think she left maybe 2 cm at most of the stem on the leaf. I put it in seed starting mix, I think one is growing roots. Here is a picture, I cross posted this in the "houseplants" forum before finding this one and they asked for a picture. How do I care for it as it grows? I water most of my plants the same, 1-2x per week depending on how dry they are at the moment.

Looks like a beefsteak begonia. I don't think I would bury any part of the leaf - it may set up rot. I wonder if I am seeing some rot already starting or something else.

Keep it in a small pot until it has almost filled the pot, then move it to a bigger pot. Water normally, don't keep it waterlogged but don't let it dry out either. Keep in a warm place under lights or filtered sunlight.

Here is an example of rooting a wedge cutting.

Thanks! Should I cut off the part of the leaf that is in the soil then? I couldn't put the stem in without the leaf getting in it. I noticed last weekend I had overwatered it, it was soggy, oops. So I held it up til it dripped out and even squeezed the pot a bit to get more water out, and now it seems ready for a light spritz maybe. Do they like to be sprayed? At this stage I usually spritz right into the soil, but periodically I spray over my spiders, and that is where it is sitting. Thanks for your help!

Spritzing is good. If you can trim away part of the leaf in the soil that may be helpful too.

When you stick a whole leaf just lay it flat like the photo above.

Cool! Thanks!!