Fusion hair extensions are quite a popular method. There are quite a few different types of fusion hair extensions but they can generally be bundled into two main types, cold and hot fusion.
About Hot Fusion and Cold Fusion Hair Extensions
Hot Fusion Hair Extensions: Hot fusion involves attaching the extension to the natural hair with hot glue. You can use a fusion connector to heat the glue before attaching or many simply use a hair straightener. To get a more in-depth guide to adding these extensions, check out this previous post on adding fusion hair extensions.