The brutal part of the process cames next. Tear or cut the plant into reasonable sized pieces for replanting. Think of it as helpful surgery. In my case I intend to put about three back into the pot. So chunks of root about three inches across are about right. I put some in the tray and a a couple in assorted containers on the bench.
Each chunk needs a good set of roots and healthy growth on top. Most plants will separate into reasonable sized pieces almost on their own. Begonias work that way so just look for a section that looks like it could be a separate plant and break or cut it away.
This one could be broken again into two pieces. Look at the stem that has grown horizontal and rooted to form a new plant. If it was cut then the two would be easily separated.
This section has a lot of new growth and young looking stems and would be a good candidate for replanting.
I have chosen it and two others to make their homes back in the pot. It would probably be better in the long run to just choose one but my wife will like this pot filled as soon as possible for the next season so I will use the three and let them grow together.
The remainder look pretty good as well but we have enough begonias for the house and patio with this one. The rest will make good compost. They could also be potted up for gifts as well.