A digital scale is recommended, with a known degree of precision. Given that trigger pull is measured in ounces, if the precision of your scale is somewhere in the pounds range, it won't provide useful results.
Place firearm muzzle-up, as straight as you can.
Tare out scale so it reads 0 when the firearm is on the scale
Watch the readout carefully as you slowly add more pressure to the trigger.
When the trigger breaks (dry-firing the gun), you will record the last weight which the scale indicated.