This benchmark could be a project progress check-in, a product, a quiz, or any combination of these things.
In a more traditional unit, each piece (what I am calling a chunk) ends in a quiz. The quiz is usually summative in nature, as it tests the students' knowledge at that point, and students move on to the next chunk regardless of their score.
In strictly Personalized Learning, each chunk ends with a formative assessment. If the student cannot pass this assessment adequately, they are not allowed to move on. Instead they are given additional time and supports before the re-attempt the formative assessment.
What to Do
Look at your list of learning objectives, and think about how you usually teach the unit. Remember that our goal is to make sure that every learning objective is taught, developed, and assessed. Then, give yourself permission to do it completely different. Maybe you won't, but before moving on you need to be OK with the possibility. Depending on your teaching style, and the type of unit, you may go about this step differently.
The typical method is to put the most foundational and easiest content and skills in the first chunk. Subsequent chunks have more complex content and skills that build upon those learned in previous chunks. Different teachers may have different ideas on the best way to sctructure this sequence, however.
For some projects, students will need to learn content and skills in a different order. For projects, in addition to asking what is most foundational, ask yourself what content and skills your students need to complete the first part of your project.
And, here is the strange thing, it turns out that our students can learn content and skills out of what we would consider the "proper" order. You can teach students about projectile motion without first talking about vector components, which is definately a foundational skill.
For projects, continually ask yourself: "What do my students really NEED to know to complete their project?" It's actually a pretty good question to ask yourself regardless of what content you are teaching.