If they will be idle for 15-20 days, then you may ask them how they would like to fill this time gap productively. Let them brainstorm and come up with their own solution. I hope they are all software engineers, therefore graduates in Engineering.
Secondly, a large number of soft skills can be learned on one's own. There is no need to "teach" as such. Given that they are engineers and supposed to have a high IQ level, self-learning is always the best way to learn. You may identify the top 5 behavioral competencies and ask them to master these in 15 days. Let them refer to any book or web resources on their own. The knowledge of the following competencies can be acquired individually:
- Listening Skills
- Questioning Skills
- Decision-Making Skills
- Conflict Handling Skills
- How to give feedback and how to receive feedback
- Interpersonal Skills (with an emphasis on how to criticize and how to accept criticism)
On the 15th day or so, you may conduct a test to determine the extent to which they have mastered these competencies. Even if they have a theoretical understanding of any competency, it is more than sufficient. They can apply this theoretical knowledge when their projects begin.
Alternatively, you may suggest that they go through Stephen Covey's book on the 7 Habits. Conduct a test for each habit. Trust me, if they master these habits, they will bring about significant cultural change in the organization, as changes in their behavior will be quite noticeable. However, do not just ask them to "read." You must conduct a test; otherwise, nobody will show interest in mastering anything.
By employing this method, you will be able to foster a learning culture within the organization, which is far more important. However, to implement all this, you should be competent enough to handle it, lest it causes more harm than good.
If you choose this approach, you will likely find that you have saved a lot of resources in training them.
All the best!
Regards,
Dinesh V Divekar