If you haven't already thought about the skills required to successfully understand the standards you are covering then you need to do so now, before the project starts. While you are monitoring groups you will see that some students will not have a firm grasp on what you are expecting them to learn. There will be learning gaps. There will be misconceptions. You need to step in a teach to fill in the gaps. Here is another great thing about PBL - you really can do individualized instruction while the rest of the class is working.
What we will do is get a feel for how many students are struggling with the concept. Is it most of the class? Is it half the class? Is it 3 or 4 students? If you are noticing half of the class struggling with the concept then you need to hold a whole-group session. It won't hurt those who do understand to hear it one more time and it may eliminate some misunderstandings that you didn't catch in your walking around the classroom. If it is just a handful of students you should announce that "in 10 minutes I will be having a workshop on _______ at the front of the room." You then need to make sure that the students, who you know are struggling, attend. A simple walk by with a quiet "you need to be at that workshop," should be all you need to do to get that student to the workshop.
The next thing you will need to do is assess the students for understanding. Just because it's PBL doesn't mean quizzes and tests go away. If this is a time when you need to have a major test then you need to fit that into your plan. If it is just a quiz then you just have to remember to give your students time to finish it. One thing I like to do is have a test over the material being learned during the PBL unit on the day before their presentations. That way the students will know what you consider the most important elements of the learning and they won't be surprised by the questions being asked of them during their presentations.
A less formal assessment of knowledge can come in the form of a poll or a "ticket out." There are multiple ways of gathering the poll information from clickers to phone apps to google forms. Or, you might just have them complete the following sentence stems and hand them to you on the way out of class: (1) What I now know about (the subject) is ______________ (2) What I would like to know more about (the subject) is ______________ and (3) What I am still confused about (the subject) is ________ . That simple step really helps you plan your next day's work day and workshop requirements.
Something that you may not anticipate in a PBL classroom is students learning things that you didn't know or that you learned a long time ago when you were in college. But wait! YOU are the one in the classroom with the most knowledge of the subject matter. That does not mean that you know everything about the subject matter. In PBL there will be groups that will go way beyond what you have required of them. They might just learn something that you aren't comfortable teaching because you've never had to teach the subject to the level they need. This is when you need to suck it up and say, "let's learn this together." There's nothing wrong with asking that group to stay after school or come before school or come during your lunch so you can all go over the information together. That small amount of time will be the most important thing you can do for your students and yourself.
I'll follow that last paragraph up with a word of advice - listen to your students. They know when a PBL project is working and when it isn't. They know if you blew it and didn't consider all of the possibilities. They also know when you have hit a home run and they have not only learned what they were supposed to learn but they have gone way beyond what you had hoped and they enjoyed the learning process. That's right they enjoyed learning. Isn't that what you like to do?
Don't you hate boring, meaningless, professional developments? Isn't it fun to walk out of a PD and say "wow I really learned something and I can use it in my classroom right away?" That's the same way our students are about their learning. It needs to be enjoyable. It doesn't have to be "fun" every day but the students should want to come to class and learn. That is what makes a PBL classroom so much fun to teach in. So listen to them. Ask them for feedback after you finish a project. Ask them for concrete things that went well or didn't go well at all. Then, when you get to this point next year, you can either create a whole new project to cover those standards or you can tweak this project so that it is more effective.
There, you are ready. You will have great days. You will have not so great days. But I can guarantee that once you and your students have become PBL learners you won't go back to the old way of teaching. You, and your students, are about to discover that it is OK to learn stuff. And, you might just have some fun along the way. Now roll out that Entry Event and get going.