Starting is probably the hardest thing. In reality, you can begin anywhere you want. The important thing is that you do something little every day, or at least a few times a week. In 1-2 years time your posture will be impeccable. Few points off the top of my head:

• The idea is to learn to be your own chiropractor. For example, if you curve your spine backward all the way from hips to neck, you will be able to crack your verbetrae by squeezing your back/latissimus together hard. Over the past few months I have produced somewhere around 70-100 loud cracks and my scoliosis has practically gone away for good. Similar cracks can be produced all over the body, including the skull, and nearly always they come with permanent increase in range of motion
• Pay close attention to how your better side performs movements. Then try to find how you need to position yourself in order to replicate these movements with the weaker side. Try to tense, flex and rotate your muscles with all your willpower until they begin to spasm. This returns the inactive parts of the weaker chain to your conscious control and gives them some baseline muscletone that helps keep you upright. In every movement, there are both stabilizing and guiding muscles. Try to become aware of both.
• I suspect there is no such thing as one leg being smaller than the other. Your other leg is supported by the hip, the other is not, which creates the illusion of differing lengths. Your foot arch may be more collapsed on the weaker side too, increasing the functional difference. My mother was told a similar story
• The motion of sucking/pushing against the anterior palate is connected all the way down to the groin area. Suck your stomach in, then try to connect the sucking motion with the tongue & palate. This requires hard effort to maintain in the beginning, but when done correctly it provides huge postural support. 
• Skull, spine, shoulders and hips should be aligned between your ankles. This is your center of gravity that gives you the greatest possible stability for all movements. If one of these ends up moving too far away from your center of gravity, you are not probably engaging your body in a wholesome way.
• Asymmetry is always LARGER THAN IT SEEMS. By the time asymmetry becomes visible, your body is significantly imbalanced.