Identify the key in which you are soloing. Since the Phrygian mode begins on the third note of the major scale, knowing the key you are in will allow you to determine which major scale you will be using notes from.
If you are in the key of B, for example, and want to play the B Phrygian mode, you will need to know that B is the third note of the G Major scale.
When you have identified your key and which scale you will be using notes from, you are ready to begin. In our example, if you are playing in the key of B, then B will be your root now, and the remainder of the notes you play will be from the G Major scale.