Step 1: Find valence e- for all atoms. Add them together.

P:5
H:1x3=3

Total=8

Step2: Find octet e- for each atom and add them together.

P:8
H:2x3=6

Total=14

Step3: Gives you bonding e-. Subtract step 1 total from step 2

14-8=6e-

Step 4: Find number of bonds by diving the number in step 3 by 2(because each bond is made of 2 e-)

6e-/2= 3 bond pairs

Step 5: Find the number of nonbonding (lone pairs) e-. Subtract step 3 number from step 1.

8-6= 2e-=1 lone pair

Use information from step 4 and 5 to draw the lewis structure

Lewis dot structure of PH₃

Alternatively a dot method can be used to draw the lewis structure.
Calculate the total valence electrons in the molecule.
P:5
H:1x3=3

Total=8

Put phosphorous in center and arrange hydrogen atoms on the sides.Arrange electrons until phosphorous get 8 electrons.