NOTE: It is hard to write a solution for this, as it is pretty much a test of devotion and/or skill. Below, I simply outline what you need to do. As everyone pretty much knows, Cathedral and Little Guitars and fairly easy songs, and I did not go into details with them.
This is the easiest of the four songs, and you shouldn't have trouble with it. If you do, you're going to encounter a lot of problems when going for this achievement.
Though the intro solo is a bit difficult, it is nothing to be intimidated by, as it is, in my opinion, the only difficult part of the song. Past the solo, the song is just a repeating verse/chorus. Unlike the other three songs, Little Guitars is not a guitar solo, and is not that difficult (outside the intro solo, like I said.)
Score Goal: 80,000
This is where the achievement gets drastically hard. In my opinion, Spanish Fly is the hardest song to five star, because the tapping section is a bit awkward. If your going to five star this, you must be able to play two sections flawlessly. The first is the tapping section. This must be practiced. I practiced for hours before eventually getting it down (devotion scored me the achievement, though.) You must also be able to play the ascending and descending notes near the beginning of the song. Play the intro, building up star power (THIS IS NECESSARY!) Use your star power in the ascending/descending notes. If you play flawlessly (which you must) you will be well on your way to the five star. Then, it is just the tapping. For the five star, it is necessary to obtain the two star power phases during the tapping sections. Once you get it, use it right away, and you should be able to pull off the five star. With this method, Spanish Fly is very easy to five star.
Score Goal: 117,500-118,000
Eruption is not as hard as Spanish Fly, in my opinion, because you can pretty much bomb the first half of the song and still accomplish the five stars for the achievement. To get the five stars, simply load up your game, go straight to practice mode, and practice the tapping sections until you can play this flawlessly. Don't be worried if you don't get it right away. It took days to learn this!! Once you can consistently play the tapping section flawlessly, your pretty much good to go. Obtain every star power phase (the one near the fast notes is hard to pull off at first, but easy with a little bit of practice) and save it all for the tapping section. Make sure you whammy the sustained note of the star power phase after the alt-strumming part, as it will net you A LOT MORE star power that you would have originally. When the tapping section comes up, wait until you achieve a 4x multiplier and activate star power. This is where your training will kick in. Play flawlessly for as long as you can with the 8x multiplier, and you should be able to pull off the five stars.
NOTE: I said before that you can "bomb" the first half of the song. Obviously, that's a little exaggerated. You must be able to hit a majority of the notes in the first half, and pull off every star power phase. But, do not restart if you miss a few notes or break multiplier during the first half. As long as you perform extremely well in the tapping part, you shouldn't have a problem.