Here are the steps that I follow when drawing a Lewis structure.
Decide which atom is the central atom in the structure. That will normally be the least electronegative atom (P).
Draw a skeleton structure in which the other atoms are single-bonded to the central atom — a P atom with three H atoms attached to it.
Draw a trial structure by putting electron pairs around every atom until each gets an octet.
Count the in your trial structure (8).
Now, count the you actually have available. 1 P + 3 H = 1×5 + 3×1 = 8.
The trial structure has exactly the same number of electrons as we have available. Therefore, the trial structure is the correct Lewis structure.
