Within the topic you want to index, use the <idx> element as shown:
Or, the short form:
Or, to control how the entry is sorted, use the <sort> subelement as shown:
Where keyword is the text you want to display in the index and sortkey is the text used during sorting.
The <idx> element can be used anywhere within the topic. Neither the keyword nor the optional sortkey are displayed in the topic.
Here's the start of a topic with two keyword index entries:
<s1 id=getting-started>Getting Started with Helpview <idx>starting Helpview<\idx> <idx> Helpview, starting<\idx> Welcome ... . . .
The following example indexes the + (plus character), putting it in the keyword index where the word "plus" would appear: