Summarize:

Comb the section of hair to be trimmed, and hold it firmly between the index and middle finger of your nondominant hand. Use the razor tool in your dominant hand to gently trim away the excess hair in short strokes. Using a razor tool instead of shears will help keep your ends from looking too blunt. The effect should be feathered and soft to help your true hair blend with the extensions. It's best to trim conservatively at first. You can pause and inspect your first attempt in the mirror. Go back and trim again as necessary until it looks right to you. Then, take the brushed hair in your nondominant hand and grip firmly between your index and middle fingers. You should be holding the hair about 1 inch (2.5cm) above the ends. Notice any extra long pieces that are out of line with the overall shape of the new ends. Angle your shears so the tips are just barely pointed into the line of hair between your fingers. Trim any extra long stragglers with the scissors perpendicular to the line of hair to even out the edge. The goal should be to create a soft, polished line. Clip the rest of your hair out of the way so you can focus on these pieces. Use a gentle trimming motion to slide down the length of the hair that frames the face on both sides. You can hold the ends to provide a little resistance if necessary. The goal is to connect and blend the sides of the extensions with your polished line in the back. Doing this should create soft, face-framing layers. A blunt or trendy hairstyle with harsh lines or that removes lots of length can be difficult to achieve on yourself at home, especially with extensions. Make an appointment with your stylist for the best result, and plan to only trim your extensions yourself. Your stylist can also help install your extensions for you, if you'd prefer professional help.
Cut off the longer section with a razor tool. Leave some length and step back to evaluate how it looks. Comb through the hair you just trimmed. Use your shears to point cut the ends. Use your razor tool to blend in face-framing pieces. Seek out a stylist for complex cuts.