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The first extension that you will measure and apply is the piece that goes around the perimeter. Take your extensions out of the package, and open one weft so that all the hair is exposed. Starting at the beginning of your perimeter braid, hold the extension in place, and wrap it around your head. Hold it to the perimeter braid the entire time to make sure it is the right length. When you get to the end of the perimeter braid on the other side, measure a little extra hair, and then trim it to fit. Place this piece of perimeter-extension off to the side, so that you don’t misplace it or confuse it for another section of hair. You’re going to be working with a long strip of extension, but you won’t be able to use all the necessary thread in a single shot. Cut a piece of your weave thread that is 5–6 feet (1.5–1.8 m) long, and use the same process as you did for your wig cap by threading it through the needle, matching the ends, and tying them together in a knot. Cut off any excess string. Work around your head, using the exact same process of sewing your weave to your cap/braids as you have been. Remember to always double knot over a fold or the beginning of a weft, to properly secure it. When you have finished filling in your head, trim off any excess that might hang over the front. When you’ve sewn on extensions to all the small braids along your part, you should be left with a space in the back that is still open. To fill this in, you’ll have to prepare a special piece of your weave. Measure the space that you have to fill, and cut a piece of your weave that is twice that length. Start with your regular double knot at the end, and then roll the weave in to create a small circle. Create an additional knot by looping around the two layered sections of weft, and pulling it tight. Continue rolling and knotting the extension until you have finished the whole piece.  When you tie the last knot, pull the needle through all of the layered wefts to secure them together. Cut the excess string off when you’re finished tying it. Turn your closure piece upside down so that the weft is facing the ground, and the hair is upwards. Begin flattening the hair so that it falls down over the top of the rolled up weft. Because of the roll that you created, the hair should fall evenly around it, hiding the weft and your sewing. It may be helpful to use a flat iron on the hair to get it to cooperate with what you’re trying to get it to do. Secure the ends of the hair with a small elastic to making sewing it to your scalp much easier. Turn your closure piece back over so that the weft is exposed, and insert your needle and thread through it to tie your regular double knot. Bring your closure to your scalp, and place it where you want it to go. Insert your needle through a braid or a piece of weft from another extension nearby on your scalp, and pull it tight to create your regular knot. Continue doing this in a circular movement around your closure piece, until you’ve gotten back to the beginning.  Cut off the excess string, remove the elastic from your hair, and you’re done! Adjust the strands of your closure piece to cover up any weft that may be visible.

Summary:
Measure your extension-hair for the perimeter braid. Prepare your thread. Finish filling in your head. Create your closure for the back of your part. Adjust your closure piece. Sew your closure to your head.