Include this script on your page and then add elements with the 'btn-igave' class and the data-igave attributes
<button class="btn-igave" data-igave-campaign="4" data-igave-token="2">Donate Ether</button>
The script will wire a click event listener to purchase the token identified by the campaign Id and token index. If web3/metamask is not installed the elements will instead be hidden.