It is important to put the minimum amount of liquid solder on the copperfoil seams. A surplus of liquid flux will bubble and splatter, leaving holes in the solder bead. Cutting your flux brush about in half, at a slight angle will reduce the amount of flux on your brush and ease the application.
Pour a small amount of flux, just enough for the task at hand, into a small container. Don't pour the leftovers back into the flux jar, it will contaminate the rest. Do not use the lid of your current flux jar, as it should be sealed at all times so it won't become sticky by evaporating.
When finished soldering, wash off the flux by washing it with warm water and a very little dish washing liquid and a soft sponge, then rinse with water. Clean your flux off right away after you finish soldering. Flux will oxidize your solder seams if left over time. It also becomes more difficult to remove when it has begun to dry.
Flux, an introduction
Fluxes, a description
The Purpose of flux
The action of fluxes