If you have access to a physical whiteboard (in your home or office), you can simply point a webcam at it and share that video with students in BB.
You can create a makeshift “document camera” by strapping a webcam to a lamp or something similar, and pointing the camera at a piece of paper.
If you have an iPad or other tablet, we can mirror the iPad/tablet screen to BB sessions. Talk to your students on BB collaborate, write/draw explanations on an iPad or tablet, mirror your iPad/tablet screen in a web browser on your computer, then share the browser’s window with students on BB collaborate, for your student to see your explanations.
Alternatively and more simply, drawing on the BB whiteboard from within the iPad is actually quite good. The BB whiteboard is small, but the drawing quality is quite okay when drawing from an iPad (with a finger). Still, we cannot erase one thing at a time.
If you have a touchscreen laptop, you can improve your writing and drawing with a stylus. There are numerous guides online for creating makeshift styluses out of common household items; Michael M. tried a few and had very mixed results. He purchased this inexpensive stylus (https://amzn.to/3degyzk) and had great results with it.
Another option for those without touchscreen devices is a drawing pad (https://amzn.to/2QojkrB). These vary widely in price; MM had decent results with a very inexpensive one. You simply plug the pad in via USB (or connect via Bluetooth, depending on the pad), open some app to draw on (a BB whiteboard, OneNote, MS Paint), and write on it like a piece of paper. More expensive pads have a built-in screen, while less expensive ones are more like a large laptop touchpad; either way, the computer treats the drawing pad like a USB mouse. (If you are unfamiliar with the concept of a drawing pad, also called a drawing tablet, this is a pretty good overview https://bit.ly/2IZ76lc)