Slashing (or Docking)
The practice of slashing or "docking" the surface of a loaf dates back to the time when European villages had communal ovens where each family would bake their bread. Marking the top of each loaf with a unique symbol ensured that every family got their own bread back. Having done this, they also discovered that a well placed cut in the dough would prevent the occasional uncontrolled bursting of the loaf as it expanded in the oven, instead allowing for a more aesthetically pleasing "ear" to bloom out of the loaf. Another benefit of this blooming is that it allows an increase in the surface area of the loaf, so there is more of the crust - which every bread lover knows is the tastiest part of the loaf.
Many home bakers find slashing/docking to be one of the trickier parts of baking, and it certainly does take some practice - but don't worry, because badly docked bread can still taste fantastic.
Professional bakers slash with a tool called a "lame", (pronounced LAH-may). The easiest way to duplicate this at home is to use a wooden coffee stirring stick (pick up a few next time you're in a café) with an old fashioned razor blade threaded on to it.
Use the lame to cut lines about 5mm deep into the top of the loaf just before it goes into the oven. Try to make fairly rapid, confident cuts rather than small fiddly ones.
You will find that wetter doughs will tend to stick to the razor blade - if this happens, try dipping the blade in water or vegetable oil before you start. A light dusting of flour on top of the loaf just before slashing also helps prevent sticking, and can look very attractive as the unfloured crust formed from the cut contrasts with the floured area around it.
The direction of the cut is important for the best bloom - cut into the dough at an angle of about 30° from the vertical, not directly downwards. Bear in mind that a long loaf such as a baguette or batard expands widthways more than lengthways when baking, so your slashes should only be at a slight angle to the lengthways centre-line. Expansion during baking will cause the slash to finish at the familiar diagonal angle across the loaf.