After that, regular exercise (basic yoga might help with breathing and posture development for the meditation;) eat well (no refined sugar, cut out processed foods, etc.)
After all of that, get your ass to sit the fcuk down and focus, consistently.
It will come.
Proper breath technique:
- fill your stomach first
- next your chest
- deflate the chest
- deflate the stomach
- let the stomach deflate until you "make a fist" in the stomach (complete muscle contraction.) this is at complete exhale.
- now breath in, repeating the process.
It should take you three long seconds to breath in, and three long seconds to breath out (you know "one one thousand, two one thousand", you won't actually be timing, just make it a long smooth in-out.)
That's basic breath work.
You can practice lying on your back, though you should be comfortable with the technique in any position (sitting, standing.)
There is no "extent of it", you will push your boundaries little by little. After a few weeks you will notice some change, after a couple months you will notice profound effects. After a few years you will incorporate this into other things, after a few decades you will notice how confused and short minded others can be ;)