This wont be a 100% solution on how to get this but just some tips from a mediocre singerNow, what are 'Freestyle' vocal ratings you ask, well Freestyle is supposedly when you sing a different word that fits the song/rhythm since you might not know all the lyrics but it also helps for humming. You can tell when part of a phrase has registered as a freestyle since instead of the pie chart for the phrase filling up all blue you will see an orange section.
A little bit of orange isn't enough you need to have freestyled at least 30% of the phrase whilst also getting enough blue to fill the pie chart also so it would have been an 'Awesome' only since you have some freestyle it becomes 'Freestyle' rated. Earning one 'Freestyle' will pop this achievement mid song, however there is an achievement for getting 3 in one song so ill detail a good song for that.So the song I used for this achievement was
Prayer by Disturbed and I played on Expert though freestyle is also available on Hard.
I used a headset for my controller instead of a mic so a mic will be
easier and I hummed with the tune of the song trying my best to match the pitches on the screen and sometimes the game registered it being freestyle, now I don't think this will work for everyone but hopefully for some.
Getting 2 more in the same song will unlock
The Wingin' It achievement in Rock Band 4 worth 30 pointsEarn three "Freestyle!" vocal phrase scores in a single song.