The biggest Do of writing fantasy is letting your imagination run free – after all, it is a fantasy that you are writing. However, within your fantasy world you need to have some rules. You get to make up the rules, but … you must abide by them – and it must be believable. It helps to make notes on your fantasy world, giving it a history and peopling it with characters so that it becomes real to you. But unless you are writing high fantasy, you won’t need to reach to the level that J.R.R. Tolkein or Georges R.R. Martin went to. (Is there some spooky coincidence to them both bearing the R.R. middle initials?)
Whether you are writing high or low fantasy, or even urban fantasy – there are many creatures
that have now become cliches: dragons, vampires, and werewolves are common examples. Does that mean you can’t use them? Not at all. It means you need to find something fresh about them – forget twinkling vampires – Stephanie Meyer beat you to that in the Twilight series, plus that was the weakest point in her book, if you ask me.
This is the fun part. You get to decide what type of a world it is, whether magic is a natural resource or a treasure that only few can possess. You get to create continents and oceans and people. You are a God, but just like God put certain physical rules into place – you have to do the same. He was consistent. Gravity is something that is felt over the entire world – not just in North America or Asia. In the same way, make sure you remain consistent in whatever you choose.
The Science Fiction and Fantasy Writers of America‘s website is a resource that all fantasy writers should explore. This particular page (see link above) lists tons of questions which will help you build your fantasy world.