The simplest way to learn it is check a code of existing plugins. Below some tips.

1. Each plugin have to implement one function

   HWND __stdcall view (HWND hParentWnd, const unsigned char* data, int dataLen, int dataType, TCHAR* outInfo16, TCHAR* outExt16)
   

   hParentWnd is a dialog window handle.
   data, dataLen and dataType represent a value. dataType is one of SQLITE_TEXT or SQLITE_BLOB. If dataType equals to SQLITE_TEXT then data contains TCHAR/wchar_t-string.
   outInfo16 is an output string that will be shown in the left bottom corner. Max length is 255.
   outExt16 defines a file extension when the value will be saved as a file e.g. txt.
   
2. You can free resources in int __stdcall close (HWND hPluginWnd) - function.
3. Plugins have a priority (high priority plugins are loaded first). To set the plugin priority implement int __stdcall getPriority (). It's recommended to allow to user to change the priority via ini-file.
4. All standard plugins use ini-file to store their settings (often it has just a priority). Typical ini-file for plugin-name is below (don't forget to provide a section):

   [plugin-name]
   priority=10
   another-option=ABC
   

5. Keep in mind: Windows UI uses UTF16LE (wchar_t). It means that each symbol is stored in 2 bytes. The recommended codepage for SQLite3 is UTF8 where each symbol required 1-4 bytes. To convert betweem UTF16 and UTF8 you can use the next functions:

#define UNICODE
#define _UNICODE
...
TCHAR* utf8to16(const char* in) {
	TCHAR *out;
	if (!in || strlen(in) == 0) {
		out = (TCHAR*)calloc (1, sizeof (TCHAR));
	} else  {
		DWORD size = MultiByteToWideChar(CP_UTF8, 0, in, -1, NULL, 0);
		out = (TCHAR*)calloc (size, sizeof (TCHAR));
		MultiByteToWideChar(CP_UTF8, 0, in, -1, out, size);
	}
	return out;
}

char* utf16to8(const TCHAR* in) {
	char* out;
	if (!in || _tcslen(in) == 0) {
		out = (char*)calloc (1, sizeof(char));
	} else  {
		int len = WideCharToMultiByte(CP_UTF8, 0, in, -1, NULL, 0, 0, 0);
		out = (char*)calloc (len, sizeof(char));
		WideCharToMultiByte(CP_UTF8, 0, in, -1, out, len, 0, 0);
	}
	return out;
}
...
TCHAR* str = utf8to16("Привет мир");
_tprintf(str); // do smth with UTF16-string
free(str); // release memory