I admit my first impression to the question was "OMG! Two instrument pilots, neither of which knows the procedure to start an IFR flight!"
But, in truth, the procedures for practice approaches under IFR can differ pretty widely from place to place. "Local IFR" where one simply calls CD and says they wanna go up and practice can be standard in some areas, while the formal filing of a full flight plan may be required in others. On top of that I've noticed that the problem of not knowing what to do and how is worse in areas with normally clear weather where practice approaches are rarely done under IFR anyway. It's not at all uncommon for people to get rusty.
To answer your question, it depends. Filing before hand is never improper. Not filing beforehand may be standard in some places. And pol-up clearances can be workload-based.
If you're really rusty you =want= to practice filing real flight plans, and getting real pop-ups, and getting clearances on the ground with void times and...