This is not a casting manual but this is!
If you are going to learn how to fish then first you must learn how to cast. Casting is a skill. Skills require knowledge and practice.
Professional and qualified tuition is by far in the best way of obtaining this skill.
Fishing large stillwaters with any degree of seriousness, will force you into becoming a good caster. There are many times when you need to be able to cast a very long line indeed.
I have yet to see an efficient double haul (or even a nice roll cast) which has been self-taught. All the good casters I know have had lessons, myself included. Do yourself (and me!) a favour and invest some money in your casting.
Once you have acquired a smooth double haul technique with nice tight loops, the ability to put the fly in the perfect position with slack line casts and have mastered the single Spey cast, then you have become a very rare caster indeed.