Each gang has tagged 25 locations on their territories, so you need to tag 75 locations troughout Stilwater. To tag, enter the trigger Field and press Y. You must input commands showing up on screen, don't make mistakes or you'll have to start again. If you haven't finished the game yet, your gang notoriety will increase as you input commands. You can see if you already tagged a location if there is a Saints logo on it. When you complete the 25 tags of a gang, a screen show up telling you that your sprint gauge increased.
Update (thanks System of a Dom) :
The quickest way to find all the 75 tag locations is to use this map :
Choose to display only Tags then click on the tag you want on the map. You can mark which tags you have done/checked.
Edit (3 years later...)
Unfortunately msxbox-world seems to be closed so you can't use its interactive map anymore. I didn't find another interactive map so if you're still trying to get this achievement check this page http://www.xbox360achievements.org/forum/showthread.php?t=87...
but sorry there're no screenshots to help like with the old map... You should download the map and edit it with paint to check the tags you did if you dont want to have to go back to all the locations because you're missing just one tag.