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

N:5 
F:7x3=21

Total=26

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

C:8
F:8x3=24

Total=32

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

32-26=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.

26-6= 20e-=10 lone pairs

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

Lewis dot structure of NF₃

Alternatively a dot method can be used to draw the lewis structure.
Calculate the total valence electrons in the molecule.
N:5 
F:7x3=21

Total=26

Put nitrogen in center and arrange fluorine atoms on the sides.Arrange electrons until both nitrogen and fluorine get 8 electrons.