Select the polygon lasso tool & match your settings to mine below:
Zoom in if needed to get close up on the teeth in the picture you're using.
Use the polygon lasso tool to make the tip of fangs on the teeth like this:
Select the smudge tool & match your settings to mine below:
Using that tool smudge the teeth into your outlined fangs.
[Extra Tip: to define the teeth more you can select the brush tool,
make a new layer (Layer - New - Layer) & fill in your fang tip outlines with
the color white (hex code: FFFFFF).
On that new layer lower the opacity so the fangs blend in more with the other teeth.]
Go to Select - Deselect, add any other effects you want then you are done!