As someone who's only good football skills is good shots with both feet, here's my approach once I decided to improve my weak foot.
Go to a football field, go to the penalty spot and start shooting with your left foot. This will be awkward and you will hit like a 4 year old. That's fine, you will get the feel for it after a while.
Once you're comfortable doing the motion with your left foot, start aiming in the corners of the net and call your shots. Rinse and repeat for the other techniques. The reason you have a weak foot is not because it is weak, it is lack of technique. Once you start to practice using it, you will get better at it.
There's nothing more to it, practice makes perfect.
How to Develop Your Opposite Soccer Foot This article from James Holloway on LiveStrong suggests a similar process. It also contains a little more information on how to improve.
Shooting Drill
A simple drill can improve shooting with either foot. A small group of players or coaches is needed. A player begins by running and receiving a pass from a player to the right of the goal, then shooting. Without waiting, she continues running to the left to receive a pass from a player standing by the side of the goal. The first player then shoots with the left foot. The drill should be reversed for players whose dominant foot is the left.
Patience
Most people spend their lives favoring their dominant foot. This isn't limited to the soccer field but occurs in all walks of life. As a result, the opposite foot, and the opposite leg in general, are not going to be as strong or coordinated as the dominant leg and foot.