<header class="header">
  <div class="header__logo">
  </div>
  <div class="header__menu">
    <a class="header__menu-item" href="/something">Something</a>
  </div>
</header>

.header {
  //styles

  &__logo {
    //styles
  }

  &__menu {
    //styles
  }

  &__menu-item {
    //styles
  }
}

header {
  //styles

  .header-logo {
    //styles
  }

  .header-menu {
    //styles

    &__menu-item {
      //styles
    }
  }
}