The most important things you need are an iron and a tailors ham. A tailors ham is just a fabric tube that is filled with saw dust. You can either buy one or make one yourself. You can find a great tutorial from Tilly and the Buttons here. I have to admit, that I bought mine because the tight stuffing scared me off (and that although I have tons of pet litter from my guinea pigs that I could use). The ham is important because you are going to press a curvy and shaped area and you can find a similar shaped area on your tailors ham - so perfect for pressing! If you do not have the budget to get a tailors ham you can also tightly roll a bath towel to a sausage and use that (that's what I did in the beginning).
So let's start to press you dart (you will need a lot of steam and if you have a delicate fabric a pressing cloth):
1. Press your dart flat, making sure you do not press the dart point – you don’t want any creases here!
2. Place the fabric right side down on your tailors ham, finding an area that resembles the shape of your dart.
4. Place the fabric wrong side down on the tailors ham thereby smoothing the dart toward the centre back or front (for waist darts) or up (bust darts) with your iron. Avoid pressing over the dart fold as this might leave an imprint on your fabric.