<script src="/path/to/masonry.pkgd.min.js"></script>

<div class="grid">
  <div class="grid-item">...</div>
  <div class="grid-item grid-item--width2">...</div>
  <div class="grid-item">...</div>
  ...
</div>

.grid-item { width: 200px; }
.grid-item--width2 { width: 400px; }

$('.grid').masonry({
  // options
  itemSelector: '.grid-item',
  columnWidth: 200
});

var elem = document.querySelector('.grid');
var msnry = new Masonry( elem, {
  // options
  itemSelector: '.grid-item',
  columnWidth: 200
});

// element argument can be a selector string
//   for an individual element
var msnry = new Masonry( '.grid', {
  // options
});