Keeping in mind that all extensions are developed by taking the PortSwigger-provided structure (which was previously shown) as the code base, the entry point for your extension is as follows:

void registerExtenderCallbacks (IBurpExtenderCallbacks callbacks); 

If you want to call your own extension, you will need to use the following method:

callbacks.setExtensionName (Your extension name); 

The following code shows the byte utilities. They are useful for managing strings, searching substrings, encoding, decoding, and more:

int indexOf (byte[] data, byte[] pattern, boolean caseSensitive, int from, int to); 
String bytesToString(byte[] data); 
byte[] stringToBytes(String data); 
String urlDecode(String data); 
String urlEncode(String data); 
byte[] urlDecode(byte[] data); 
byte[] urlEncode(byte[] data); 
byte[] base64Decode(String data); 
byte[] base64Decode(byte[] data); 
String base64Encode(String data); 
String base64Encode(byte[] data); 

The following code is used to tell Burp Suite what your extension is managing:

void IBurpExtenderCallbacks.registerExtensionStateListener(IExtensionStateListener listener); 

The method to save states is shown in the following code:

void IBurpExtenderCallbacks.saveExtensionSetting(String name, String value); 
String IBurpExtenderCallbacks.loadExtensionSetting(String name); 

Finally, this is the method that you can use to manage files:

ITempFile IBurpExtenderCallbacks.saveToTempFile(byte[] buffer);
 byte[] ITempFile.getBuffer();

If you want more details about the Burp Suite API, or even some examples of its use, check the official site at https://portswigger.net/burp/extender#SampleExtensions.