index.html

<div class=”grid-container”>
	<nav>
     <ul>
       <li>Menu item 1</li>
    </ul>
	</nav>
	<main>
    <h1>Hello world!</h1>
	</main>
</div>

main.css

.grid-container {
    display: grid;
    grid-template-columns: repeat(12, 1fr);
}
nav {
    grid-column: span 4;
}
main {
    grid-column: span 8;
}