<div class="container my-5">
  <div class="row">
    <div class="col text-center">
     <!-- Button trigger modal -->
      <button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModalCenter">
        Launch demo modal
      </button>

      <!-- Modal -->
      <div class="modal fade" id="exampleModalCenter" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true">
        <div class="modal-dialog modal-dialog-centered" role="document">
          <div class="modal-content">
            <div class="modal-header">
              <h5 class="modal-title" id="exampleModalLongTitle">Modal title</h5>
              <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                <span aria-hidden="true">&times;</span>
              </button>
            </div>
            <div class="modal-body">
              ...
            </div>
            <div class="modal-footer">
              <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
              <button type="button" class="btn btn-primary">Save changes</button>
            </div>
          </div>
        </div>
      </div>
    </div>
  </div>
</div>