You can start with evaluating the final design work of others (an existing product, for example), work out the positives and negatives and use that to formulate a problem/situation to work to.
You can start with a wide range of concepts on a theme and with the feedback from clients / interested parties, write specifications that can guide the design brief.
I am guilty of always working this process in a linear manner in the classroom, starting from a situation / problem and working through to the end evaluation. It might be interesting with the seniors to mix it up and start in the middle, with some product testing at the start of a project. The Level 1 mobile phone project coming up later in the year might be an ideal place to give this a go and that group has a really good "give it a go" attitude.