If you are in the process of making your own clip-in hair extensions you have two options as to how you will attach the hair weft to the snap clips. You can either sew the the snap clips onto the top of the hair weft with some strong thread or you can use a hot glue gun. If you are using synthetic hair you don't want to use a hot glue gun because the synthetic fibers can burn or melt. You should never try to use a glue gun to attach an extension directly to your natural hair.
Turn on your glue gun so it can start heating up. Take your hair weft and find the top where all the hair is gathered together. The number of clips you need depends on how wide your extensions are. You will need to space all your clips about two inches apart. Take your snap clips and make sure they are all facing toward you so you don't accidentally glue the part that is supposed to open.
Snap clips usually have holes in them where you would thread a needle through if you were sewing your weft to your clips. When you begin applying your glue, stay away from these holes and make small and neat lines of glue. Immediately stick the snap clips onto the hair wefts in the placement that you determined, about two inches away from each other. Make sure you have one at either end of the top of the weft, no matter what. Let them cool completely before you install them in your hair.