Press the barbell so that it’s a couple inches over your head and then lower the barbell behind your head. Then press the barbell back up so that the bar is a couple inches over your head and bring the bar back to the front. This would constitute one rep.
I recommend that you start off using just the empty barbell and work up to doing 100 total reps. (i.e. 10 sets of 10 reps, 5 sets of 20 reps, 4 sets of 25 reps, etc.) Just do as many sets as it takes to compete 100 total reps.
Now this is quite challenging, even when just using the empty barbell. Most people don’t have muscular endurance and work capacity to perform 100 total reps. So if you can’t perform 100 total reps at first that’s ok, just work within your means and strive to get more reps each workout.
When you can perform 100 total reps consistently then you can start adding weight to the bar and pyramid up in weight with each set and lower the reps like you would for normal shoulder presses and really push yourself with the Bradford Press. But just start out using the empty barbell for high reps and master proper form and technique first.