1. Start small. Use GS to fill out larger gaps. Then successively try making larger and more complex things.
2. Don't view a part as a whole, but as a sum of parts. You'll find it slightly more difficult to plan, but easier to make them from GS.
3. Always set challenges to yourself. Things that will push your skill beyond its current limits.
4. Don't be hasty, take your time working. Many were the times when I was too eager to work on another part of detail on my new conversion and in the result ended messing up what I've done before.
Four years ago I knew jack sh*t about working with GS. After a lot of training, the skill has greatly increased. Maybe it took me a whole year to resculpt a Callidus Assassin the way I wanted her to be (most of the time consumed by lengthening that damn braided ponytail) - but I was successful in the end!
Wish you the same - or higher - rate of success with your own GS experiments!