Teaching involves multitasking. You almost always are doing more than one thing at a time. A useful skill is the ability to talk and write simultaneously. While some information is helpful to write on the board prior to the start of the class, I recommend writing on the board as the information becomes necessary to your presentation. The writing then supports what you are conveying. After all, you are the teacher, not the board. Don’t let the board do the talking for you. Of course, talking and writing can be like walking and chewing gum. (Americans joke that it’s hard to do both at once.) So don’t prattle on, but on the same token, don’t write in silence for a long period of time. And step back to do a quick spell check. It’s easy to skip a letter or transpose a couple others when you’ve been talking all along.
Remember that what you write on the board doesn’t have to become the equivalent of a transcript of your lesson. Full sentences aren’t always necessary. You can use concise phrases that still have enough content to be meaningful.
Summary: Keep a balance between writing on the board and talking. Let the visual presentation support your oral explanation.