Do enough exercise to sweat a lot.

Dry your armpit with a blow dryer using cold air.

Right away put a piece of tissue paper in your armpit.

By using a marker,make a mark through the tissue paper onto the skin of the first area that gets wet.

Then remove the tissue paper.You should see the mark on the skin.

Apply a pea-sized amount of gel directly on the marked area (usually this area is on the hairy part of the skin.)

See also starch stain method.