I'd like to learn to use shellac. From what I've heard it's one of the more difficult finishes to achieve. Why? I plan to practice on a few pieces before I use it on something I really care about. What should I be watching out for?
I am very much an amateur woodworker, but I love shellac. If you are after a perfect finish, then yes, it is hard to get right.
If you are after something good enough (As I am), then it is really easy. You sand it back smooth (I use 180 grit) then start with light coats. You let it dry, sand it again and repeat. After 4-5 you should start to feel and see the smoothness. A few more coats and you have a high gloss finish.
On a nice sunny day, you can get 2-3 coats an hour so it takes no time all all. The real trick is to use light coats.
Additionally it is pretty cheap. The flakes are ~ $30 AUD and a 4L bottle of metho is $5. A bottle of flakes generally lasts for quite a few work pieces.
From what I've heard it's one of the more difficult finishes to achieve. Why?
That's a lot like saying that spray paint is one of the more difficult finishes to achieve. If you're finishing a Rolls Royce with half a dozen coats and lots of polishing, sure. But you can also just spray on a coat or two and call it a day.
So it is with shellac. If you want the deep, rich, glossy finish created by French polishing, there a lot of work and more than a little skill involved. That would be appropriate for fine furniture or a musical instrument. But if you just want a good looking, non-toxic finish that's easy to repair for, say, a baby crib, you can wipe on a coat or two of shellac and do a little sanding with very fine paper.