The Beatles Rock Band

To get gold stars you must play on Expert difficulty, if you're playing in a band then EVERY player in the band must be on Expert difficulty to be able to earn Gold stars.

Gold stars are usually around 1.5 times the 5 star score, so for example if the song is 5 starred at 40,000 points then the gold star cutoff will be around the 60,000 mark.

You need to gold star the following for this trophy.
Back in the U.S.S.R
Can't Buy Me Love
Get Back
Helter Skelter

They can be attempted on any instrument and as long as you Gold star on one of the instruments for the song, it will be ticked off on the Trophy menu.

Bass is notoriously harder to Gold star than other instruments because of the fact that its multiplier goes up to 6 so you need a higher average multiplier overall to attain Gold stars, if you mess up more than twice on Bass (especially when using overdrive) then chances are you won't gold star. A few songs in the Rock Band series have been known to be impossible to Gold star on Bass for some songs, this is especially true for "I Feel Fine" on this game which is needed for another trophy.

However on Guitar, you have more room for error but the charts will tend to be more difficult with the addition of guitar solos and chords.

If you play in a band, the gold stars can be easier to obtain because if you all use overdrive simultaneously then you will get "x4" band multiplier with 2 players, "x6" with 3 players and "x8" with 4 players, this can make gold stars so much easier to obtain.

Credit and thanks to rockband.scorehero.com for their leaderboards to look up lowest Gold star scores. Please note that these are solo scores so if you're playing in a band then these will be of no use to you.

Back in the U.S.S.R
Guitar: Approx 114,000 points
Bass: Approx 95,000 points
Drums: Approx 130,000 points
Vocals: Unknown but believed to be about 160,000 on Xbox 360 version according to TA (Credit to Xebu on TA)

Bass tends to be easier on this according to some, Guitar is not a bad choice for this one either.

Can't Buy Me Love
Guitar: Approx 113,000 points
Bass: Approx 72,000 points
Drums: Approx 106,900 points
Vocals: Approx 117,000 points

This one should be the easiest song to get, if you can't get this one then you aren't ready for the other 3 songs yet.

Get Back
Guitar: Approx 100,000 points
Bass: Approx 104,000 points
Drums: Approx 121,000 points but could be as low as 120,000
Vocals: Unknown but believed to be 103,000-106,000 on Xbox 360 version according to TA (Credit to Xebu, Gjot, jimmy vanzetti, JernauGurgeh SF & stretters87 on TA)

I personally did Get Back on Bass, the guitar chart was too difficult to be consistent on.

Helter Skelter
Guitar: Approx 173,900 points
Bass: Approx 178,000 points but could be as low as 177,000.
Drums: Approx 149,000 points but could be as low as 147,000
Vocals: Unknown but believed to be 151,500 on Xbox 360 version according to TA (Credit to Xebu)

This one will be the most difficult one, there isn't an easy instrument for this song. Bass chart has changing speeds in the outro which can wreck a gold star run. Guitar is easier than Bass to get gold stars personally, it's still difficult though especially the chords in the slow down section which are hard to get the timing on. There are 3 note chords in the outro which are essential to use overdrive on.

The four songs that you will need to get gold stars on are as follows...
Back in the U.S.S.R.
Can't buy me love
Get Back
Helter Skelter
To get gold stars you will not only need to play near perfect runs of the songs listed but also deploy beatlemania at the right times as well. Pick the instrument that your best at and keep playing it until you get gold stars.