Here’s what I did:
Pick a smooth air day (minimal up/down drafts). Pick a safe altitude. Pull the power to idle, and stabilize the airplane in a 65 kias glide. Record the vertical speed (VS). Repeat at 70, 75, 80,……, 110 kias.
Now go home and plot the VS as a positive number, against KIAS. Draw a smooth curve thru the data. The lowest point of the curve is minimum sink speed.
Now hold a straight edge on the x-axis, anchor the end at the origin so it can pivot but stays on (zero, zero). Rotate the straight edge upwards (keep it pinned on the origin) until it just touches the curve. That airspeed is best glide speed, and is also (lift/drag)max speed. If you want to find best glide over the ground with a wind, just move your straight edge anchor point 20 knots (kias) left if there’s a 20 kt (ias) tailwind, rotate up until you touch the curve. Move the anchor point to the right for a headwind. The ratio of airspeed to vertical speed (use the same units!) will give you the glide ratio. Finally, if you were below gross weight, multiply your best glide speed as tested by the square root of the ratio of gross weight to as-tested weight, to get best glide at gross.