For everyone lesson we have had the students draw their design first before building. So for this lesson we wanted the students to draw a design so detailed that they could hand it to another student to build it. The idea was that in engineering sometimes someone designs the project and then another person has to follow the instructions to build it.
We told the students that in groups they could design whatever they wanted, but that they should keep it simple. Explain to the students that they need to be explicit with what LEGO pieces they want to use and where they want to put them. Having the students draw the step by step process was useful as was writing out instructions. After students have finished making their design, have them switch with another group and try to build the other group's design. If possible do not have the groups communicate to each other while they are building. This emphasizes the importance of being very detailed in your design. If time permits have the two groups discuss what was built versus what was intended to be built.