You can simply add the UV powder to your gallon of resin, re cap it and shake well. We suggest giving the gallon a quick shake before each use as the UV activator will tend to settle at the bottom after a while.
You can also add small quantites of UV powder to the resin in the cup but it will be hard to measure the exact amount of UV powder to add. Too much will make the resin look yellow-ish, not enough will not cure correctly.
For reference use 7cc UV powder per gallon of resin