Courtesy of SFMTNS on Youtube
Instructions
To quickly fix, your v-brake, simply adjust the barrel out to tighten the v-brake. The further it is adjusted, the tighter the brake. If your brake lever is too soft, and the barrel is almost out all the way, then proceed with the following steps.
Grab a #5 Allen wrench and locate the cable connected to the brake.
Loosen the cable until it is loose and you are able to move it.
Pinch your v-brakes and hold the cable tight.
With your #5 Allen wrench, tighten the cable.
Next, move your wheel and you should see that your brakes are rubbing against the wheel when you move it.
Go back to your barrel, located next to the v-brake, and screw it all the way in. Your v-brake is now adjusted! To test it out, you should be able to spin your wheel, and stop it with your v-brake.