Prepare the pie crust ready for the filling. If you are using frozen pie, read directions to defrost.
In a large bowl, mix the blueberries with flour, sugar. Sprinkle lemon juice on the mixture and mix until everything coated.
Pour the mixture into one of the pie crust. Put pats of butter on top of the mixture.
Top the pie with the second pie crust. For the top crust, you can do a lattice top. If not, top with another pie crust, but you need to make slats so that the steam can come out.
Bake for 50-60 minutes. At around 30 minutes, check the pie crust. If it is getting brown, put on a pie protector for the sides.