best way to measure actual polar of a glider?

I would like to measure and compare the actual polar of my glider to
the factory polar. I would also like to compare the polar with 16.6
meter tips vs winglets. Are there practical methods for doing this
with a flight logger and normal instruments? What airspeeds should be
measured, and how many points on the curve are needed to get
meaningful results?

Any experience or suggestions for a procedure would be greatly
appreciated. The glider is a Ventus C

THanks,

Matt Herron (Jr.)