So for the most part this achievement seems pretty easy to understand but if you want to get this one legit, it is a little harder than you might think.
First of all you have to buy everything that you see in the store (guitars, finishes, songs, characters, outfits, styles and videos) if you 5 star most or all songs, you should have the amount needed to buy everything by beating expert, hard, medium and about half of easy.
Once all of that is bought there are 16 guitars/ bass guitars to buy left, you unlock 1 of these every time you beat a difficulty on career and co-op career. That equals 8 of the 16
The last 8 are the difficult more time consuming ones to get. You need to 5 star all songs on each difficulty
(4 career, 4 co-op career)
5 starring every song on the yellow setlist is all you need but doing it on expert and co-op expert will take some time and a lot of practice