Soldering Flux come in many flavors. The one like a gel come in a syringe type container, but with a thick tip and without a piston.
Take a 5ml syringe, cut the needle perpendicularly, remove the syringe piston, and fill 1/4 of the body with Solder Flux. Try to avoid any air bubbles inside the flux.
To dispense, gently press the syringe piston. Having a needle allow placing the flux on the right spot, even in the most tight PCB places. Less then half of the quantity dispensed in this picture is more then enough for one solder.
When you finish, put the needle cap, so the Flux won't dry until the next use.