Writing the custom field type is one thing, but making it available to one of JIRA's most powerful functionalities, Search, is another! When you create the custom field, you can associate the searcher to be used along with it.
In most cases, you wouldn't need a custom searcher. Instead, you can use the built-in custom field searchers in JIRA itself. The list includes, but is not restricted to, Text Field Searcher, Date Searcher, Number Searcher, User Searcher, and so on.
The first step, of course, is to determine what kind of Searcher your new field needs. For example, a text field can easily be searched with Text Searcher. A number field can be searched with Number Searcher or Number Range Searcher. You might even want to extend one of these searchers to add some extra functionality, like some special conditions or hacks you want to introduce!
Here is how JIRA has defined the Text Searcher for its system custom fields:
<customfield-searcher key="textsearcher" name...