The simple answer is to put some cold water (or even ice cubes) in a hot dish in the bottom of the oven as set out in this recipe. Too little water will not produce enough steam and too much will stop the crust crisping. So measure it accurately and if there is any water left in the dish half way through the bake take it out and reduce the amount of water the next time.
Another option is to get a water spray/atomizer and squirt water into the oven several times during the early stages of baking. Some people swear by this technique but it does mean keeping opening the oven door.
The best method is to enclose the loaf during the early part of the bake (as the dough itself gives off steam) using a covered casserole dish, metal Dutch oven or a cloche (baking dome). Then remove the lid part way through baking so that the crust crisps up.