Since this is your first PBL unit, you need to be successful. And what I mean by that is that you need to succeed AND your students need to succeed. So, I recommend a maximum of 2 weeks for this first project.
A plan for each day of the 1 or 2 weeks will help you and your students focus on what needs to happen at each stage of the process. The first day will need to have an Entry Event, the last day you will need to have some sort of presentation and you know that there will be times in the middle where you will have to have direct instruction (what we call "workshops"). Depending on the number and complexity of the standards you are encompassing you may need to plan on 2 or 3 days of direct instruction during this two weeks. So, we start with 10 days. We subtract 1 day for the first and last day and let's say 3 days for instruction that leaves about 5 days as "work days" for the students.
Work days are NOT sit at your desk and play with social media or pull out a deck of cards and play solitaire. Work days need to have a set product for the end of the day. This product might be a blog post, it might be a quiz, or it might be a written list of research that was conducted. Most students will need guidance and input from you. Most groups don't automatically have great discussions amongst themselves about the learning process. They DO have great discussions about a recent sporting event or the latest music. So plan on walking around the room listening and inserting yourself in their conversations - when needed. Don't automatically tell them what they should be doing. Instead answer their questions with guiding questions like "have you thought about ___" or "I wonder if you could _____ . "
This gets to the point that over half of the time spent in a project will, normally, be in group work. Group dynamics must be taught. This is especially true for the first couple of projects of the year. The students need to know what you expect from them. They need to know what roles they will fulfill and what requirements there are for each of these roles. So here are some ways to assign groups that I have used over the years: (a) Just a random selection, (b) Rank them academically and put first with last in a bottom-top rotation, (c) Pick group leaders and have them leave the classroom and select their groups while you work with the rest of the class, (d) Use the academic rankings to put the bottom 3 or 4 together - someone will have to rise to the top of that group.
There are many more ways to group. Just try different things and change them after each project. Your best groups, with the least problems, will want to be together for the rest of the year. Force them to work with everyone in the class so that they can see the strengths and weaknesses of every student. Some guiding thoughts can be found on Jerry Blumengarten (cybraryman)'s site under cooperative learning and collaboration as well as on the BIE site and Edutopia.
You have your plan and you have your group assignments. Now you need a "hook" to get your students thinking about what they will be doing for the next two weeks. We refer to this as an Entry Event (EE) or Entry Document. EE's can come in the form of a written document, a video, a live demonstration or skit, or a "call-in" from an expert via Skype or the phone. All of the reference sites I've already mentioned can give you suggestions but here is the archive from one of the twitter PBL Chat's we had on Entry Events.
In my last of this 4-part series I'll cover odds and ends that need to be thought about during any project. By now you are ready to do this and I look forward to hearing from you about how that first project went. You can do it! See you in Part 4.