The first, and the most important thing to get right, is the length of your curtain rods. Getting it too long or too short can both be problematic. While curtains usually come in standard sizes, the same isn't true for rods.
A common mistake in measuring your windows for curtain rods is to take the exact dimensions of the window as the intended length. Generally speaking, you want your rods to extend at least 3 to 4 inches extra.
Also, make sure to measure each window individually regardless of how similar it looks to another one. Even one inch of a difference can kill the sweeping look you are shooting for.