Some parts of writing a lesson plan are easier than others, and many CELTA trainees complain that they just don't know where to start with anticipating problems.
The first question to ask yourself is "Why is my tutor asking me to do this?" The reason is simple - life doesn't always go the way we want, and students are human beings who make mistakes and misunderstand things and sometimes technology fails and sometimes an exercise or an activity is difficult and so the lesson suddenly isn't as smooth as we hoped. The point of anticipating problems isn't that we prevent every bad outcome possible. The point of it is that we are ready to face up to the problems when they arise and we know how we're going to deal with them.
Usually, I start with language when I'm anticipating problems. What new language (grammar or vocabulary) are you teaching in a lesson? Think about what you're teaching. What makes it hard? Were any of the grammar rules hard for you to understand? That might mean they're difficult for students to understand too? Are there rules with exceptions? Exceptions can always trip people up. Imagine your students saying the new words. What might they mispronounce? Think about what mistakes you think your students are most likely to make. These are your anticipated problems.
The solutions to anticipated problems with language can be quite easy. If you have a clear written record with grammar rules, examples, pronunciation features highlighted clearly, the solution can be as simple as saying that you will indicate the rule/feature on the board and prompt the students to self-correct. You can also have some back-up questions about meaning and form ready if you do see students are struggling. Remember your context as well - you can always return to your context to "re-clarify" difficult language. If the students understand the context, then it should really help you to show them how the language works.
We don't just anticipate problems with language. We also anticipate problems with materials, tasks and learners. Here are some questions you can ask yourself:
What makes this text difficult to understand?
Which of these questions will be the hardest? Why?
Does this task require students to do more than one thing at once?
Does this task require exact pairs and what will I do if there are an uneven number of students?
What will I do if I run out of time?
What will I do if my technology fails?
Who is the biggest personality among the students? How might they disrupt the class?
Which student typically struggles with the type of lesson I'm doing today? How can I help them?
This might seem like a negative way of planning, but believe me, planning to deal with obstacles is a sure fire way of being more prepared when they come along! The better prepared you are, the smoother the lesson will be!