Sew in the wefts by looping and tying the weave thread under and over the thread in a blanket stitch method. Do not put a needle through the weft or pierce the weft - if you have to cut the wefts, seal each cut weft with an acrylic sealer.
Finger detangling is highly recommended but you can also detangle your hair using a detangling comb / brush. Gently brush your hair from the ends towards your scalp - starting from the ends as this keeps the pressure off the weft area and is a good way to maintain the longevity of the extensions.