A way to start off a lesson would be to take it step by step, slowly, especially if it is a challenging lesson. Make sure the students ask a lot of questions and you as a teacher give a lot of detail.
Also as a teacher you will need to be patient with students who are having a hard time understanding. It creates more pressure on the students having a hard time seeing that the teacher is getting frustrated with him or her. Take the student aside and try to work one on one and see what it is they are having a hard time with. It could be a language barrier, learning disability, etc. There are always ways to make it work for students, so don't give up on them.