Comb hair to remove any snarls and, holding the shears horizontally, trim to the desired length. The longer blades will allow you to make straighter, more even lines with fewer cuts.
NOTE: Always leave hair a bit longer than you'd like so you have some room to even it out if it looks unbalanced. This is especially true if you're cutting wet hair. Make sure to dry it after you cut so you will have an accurate view of the length and will be able to trim any pieces that you missed.