how to design a spur gear.

When creating a spur gear you will need to know the base diameter.
This is the diameter where the involute is struck from.
To find the PCD of the gear multiply the number of teeth by the module of the gear.

eg. 39 Teeth x 2.54 module = PCD of Ø99.06mm

In Solidworks I create an equation driven curve.
This is found in the menu Tools/Sketch Entities/Equation Driven Curve

Set the equation type to Parametric
Then set the followiing values:

X = (base circle diameter/2*sin(t))-(base circle diameter/2*t*cos(t))
Y = (base circle diameter/2*cos(t))+(base circle diameter/2*t*sin(t))

Set t2 as 1 this controls the length of the spline

This should then create 1 side of your involute form.
This is the true form that the gear cutter will produce.
Then it's just a case of mirroring the involute to produce the other side of the tooth form.

Hope this helps