<nav>
  <ul>
    <li><a href="#">nav01</a></li>
    <li><a href="#">nav02</a></li>
    <li><a href="#">nav03</a></li>
  </ul>
</nav>

nav ul {
  display: flex;
}
nav ul li {
  list-style-type: none;
}
nav ul li:not(:last-of-type)::after {
  content: "";
  width: 1px;
  height: 15px;
  background-color: #999;
  margin: 0 2em;
  display: inline-block;
  position: relative;
  top: 3px;
}