This was requested from the e-community; there are other versions elsewhere, but this is probably the simplest!
paragraph 1 should be an introduction which explains which projects you did. It can be quite short.
paragraph 2 should pick up the skill area and perhaps suggest something about your starting point with it- what skills did you have already and how were these illustrated. use an example.
paragraph 3 should talk through your use of that skill in early projects and what you learned and developed through these. Again there should be examples to support all that you say.
paragraph 4 should go on to demonstrate how the skill developed in later projects, again backed by examples, and reflecting back on how this represents moves forward for you from your early position.
paragraph 5 short conclusion
Remember it's only half an hour and you need to range across all your work!