In addition to the pun on “flow”, the “follow” language here is a nod to this song’s titular inspiration, Eric B & Rakim’s 1988 hit “Follow the Leader”. This version also takes its beat from the original

These lines are an homage to Rakim’s opening words in the original:

Follow me into a solo, get in the flow
And you can picture like a photo

See here for Justin’s bio. He’s below, with some other guy in glasses:

Even when finally a rap star, Mill would still be on probation from a 2008 jail stint. This would lead to trouble when the rapper mentioned the case in his lyrics

The “300” here is a reference to a particular set of the Black Disciples gang:

Chief Keef, and Keef’s allies Lil Reese and Lil Durk, all of whom are affiliated with the Black Disciples street gang, according to Chicago Police.

Cops say that when Keef, Reese, and Durk shout out “300” and “Lamron” in songs and tweets, they’re directly calling out to their set, which resides near 63rd Street and Normal Avenue in Englewood.

A chopper is a customized motorcycle, so when he says that he chops her like a Harley, he says that he changes her (she rides him like a Harley) and that changes her – because he’s so good and because she popped a molly.
Or he cuts her off – after they have fucked.
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O.T.F. (Only The Family) is Lil Durk’s crew/label. They are very close to the Chief Keef-affiliated Glory Boyz Entertainment crew. Says Durk:

We all the same. Only The Family and Glory Boyz Entertainment. Everybody’s family with two different names. Everybody says GBE and OTF. Everybody grew up together from the same block. Everybody knew each other since they were kids. We grew up with each other and everyone’s just got different music styles right now, doing their own thing.

[We still hang out] but everybody is usually ain’t in their city. I’ll be in Atlanta, Reese will be in L.A., Fredo will be in Connecticut, and Keef will be in Minnesota, somewhere like that. Everybody’s in different places and got different shows. Everybody’s still cool though.

Scooter here shouts out the Black Migo gang, who are based in Atlanta’s Zone 6