Grand Theft Auto: San Andreas

The EASIEST method to doing this actually requires no Girlfriend activity at all.
if you don't mind steaming through the story mode you will eventually come across a mission called "Key To Her Heart" in Las Venturas for Woozie of which you stalk a woman and sabotage her gimp in order to receive a key card to the caligulas casino.

At the end of the mission you are invited into the girls house (Millie) and a cut scene plays of which you and her reach the "final base". This counts towards the achievement because Millie becomes a girlfriend after the mission.

Happy Hunting

Once you complete the mission Burning Desire you'll save a woman called Denise Robinson. She lives down the street from Carls House and can be taken on dates every 24 hours. You can pick her up and take her around the corner to the bar and then take her home before fast forwarding 24 hours using the save in Carls house and then returning to her to take her out again. Flowers help speed up the process aswell as kisses. After 3 or 4 dates she'll ask you inside and you need to hit and that's it.

If you hit any cars while Denise is with you will most likely lose progress with her, or if she takes any damage while your with her you'll take a hit on the progress. Also return to her 24 hours after your first date to avoid her progress going down and if she doesn't like the bar, take her somewhere else.

Easiest girlfriend to get this with will be Denise, the first girl you can date after completing the mission 'Burning Desire' given to you by crash. You can just keep taking her to ten green bottles (the bar right outside grove street) and she should always enjoy it, this is also literally around the corner from her house. Something else I'd like to add, you can go to the LSPD showers and get a dildo there, save it and when you're on your date equip it, get close to Denise and you should be able to give it to her as a gift (a lil ribbon icon will show on your screen) to do this just press left on the D-Pad , you can also give her flowers which can be found at the idlewood gas station. Doing this raises her like percentage (if I recall correctly by 5 percent) with you, which can be checked in stats under achievements, once it reaches around 40 percent, she will invite you inside for some 'hot coffee' after your date, accept it, get your groove on, and you will get the achievement. Do note that the first time you leave Los Santos for the storyline Denise pretty much breaks up with you, so I'd go for this chievo before you leave LS.

If you give her a gift each date, and take her to TGB or any place she enjoys it should only take 2-3 dates. If she isn't available for a date save at the grove safe house 6 hours will pass and she may be ready for one then, if not you can continue that step till she is.

If you think I made a mistake or want to add something to this solution feel free to comment below.

Easiest way is just to play through the story and you will get this through the key to her heart misson in las venturas

After the mission ''Burning Desire'' you will unlock denise as a girlfriend, so, during night time, after midnight she will be at home, take her to a bar or to Cluckin' Bell, and after 5 or 6 dates, she will invite you to have sex press do accept, and BAAM! achievement Unlocked, enjoy your 20G.

Why Denise?
You have a Cluckin' Bell down the street and a Bar a block ahead.

If you negative vote my guide, please comment why, so do I can improve it :)

You can do this with any girlfriend but I will do it with Denise Robinson who is unlocked in the Burning Desire mission, right at the beginning of the game, to increase your points with her you need to take her to a pizzeria or to the bar, to achievement to be unlocked you only need to reach 40% progress with she to unlock the "coffee" option, she is available from 4pm to 6am once a day, after completing the meeting with her go to your house and save the game until if 24 hours pass for you to repeat the procedure, on the map she will be marked by a symbol of a heart.