1. Search for lyrics online & Paste it on Notepad
For example: on Google.com, search for "I want it that way lyrics". After you have found the lyrics, paste it on Notepad.
2. Edit the Lyrics so that it can flow smoothly.
Note: This is optional

3. Download, Install DviXLand Media Substitler at: http://adf.ly/1Sd1V
It is totally FREE

4. Open DviXLand Media Substitler, and then open:
File > Open text........ (select text file)
File > Open video...... (select MP3 file)

5. Press and hold "Apply" button to sync it. Each time you press, hold and release the "Apply" button, the time duration is accounted for 1 text line. Repeat the hold process until the end of the song.

6. Once you've finished it, click File/Save as to save it as Karaoke Lyric LRC File.
Note: The .LRC should have the same name as the MP3 file.

7. That's it. Put the LRC file in the same folder as the MP3. Open the song with any Media Players you like and Enjoy!
Note: For displaying lyrics on Windows Media Player, you must install this small plugin (256kb) http://adf.ly/5vaWn

Repeat the process for MP4 Video file. However, it is better to save your lyric file in .SRT format as below image:
How to Synchronize Lyric for MP3 Songs
How To Make an LRC (lyric) Files for MP3 / MP4 songs