I have noticed a lot of threads recently asking for help with doing smile lines...using Brisa (or any gel), so took time to see just how I do mine and made some notes because although I am no expert I'm ok at them.

Firstly the best brush I have found for doing them is Creatives Crystal Touch Petite..gel edge #6 this has a square straight edge and held at the right angle is perfect for easy application of the paint on white cos the brush that come with it can be to bulky.
Pic 1

Start very thin... with some POW on your Crystal Touch brush and the brush hairs flat, head straight for the middle of where you want your smile line and put the tip of the brush in place and then swipe straight down towards the free edge. This should leave a thin stripe as wide as your brush going from smile line to free edge. This is now your guide... because the brush is square (straight edge) you should have a straight line in the middle of your smile.
Pic 2

Then do the same again this time starting alongside the first swipe and tilting your brush slightly... and again on the other side of your first swipe.
Pic 3 & 4

Tilt your brush just a little more and do another two swipes... 1 at each corner of the smile line.
Pic 5 & 6
then cure... thats the hard bit done... it will look patchy cos of the thickness of the POW, but this is your stencil for the second coat, it's plain sailing from here.

Then with your clear coat of Brisa.
Pic 7

This is the finished smile line with a coat of Creatives Moonlight and Roses #45 to finish.
Pic 8

Like I said I am no expert but this is how I practiced over and over...and it worked for me.
Hope this is helpful to someone.