Sample taken from "Adopting the dim-sum strategy"

Example sentence using both semicolon and colon:

"They've clearly read those studies about children and second languages: Children lose their ability to learn a language fully and perfectly by age 4; more languages equates with higher intelligence."

In this sample, the colon is used to introduce. Specifically, the colon tells us that we will read the details of "those studies about children and second languages" after. The semicolon here separates the two related ideas that are found in the studies.

Here is my sentence using the same structure:

Students, clearly, need to learn more about punctuation to be successful writers: Most of them have had little practice; more chances to practice makes for a better result.

Note that Fiona Lam (and her editor, Moira Dann) have bent the rules somewhat. Normally, you are not advised to have a full clause following a colon. In this case (and for practice) I've imitated as best I can the orginal structure. Students, of course, need not select such a complex example!